columns horizontal alignment

The main problem is long text in the headers stretch the columns very wide. I've replaced the header text of the last two columns with

multicolumn{2}{c}{
    parbox{16em}{
        centeringsmallskip
        Variação Percentual do $delta_{R a_{L_{p}}}$,
        conforme variações nas incertezas:
        smallskip
    }
}

which restricts the text to 16em (roughly 20 capital letters wide).

(Note it's missing the legend. I don't know what you packages you're using for that.)

Additionally, I've made the columns equally wide (except for the long $delta ... = delta ... = delta ...$ ones). This is done by adding before your begin{document} the code

usepackage{array}

newcolumntype{P}{>{centeringarraybackslash}p{6em}}

Now instead of the c column alignment, we can also use P. This is also a centered column, but precisely 6em wide. I replace your {c|c|...|c} by {c|P|P|P|P|c|P|c}. This gives the result shown.

The full code behind this image (also on pastebin: https://pastebin.com/8StLqGUm):

% Source: https://tex.stackexchange.com/questions/598089/columns-horizontal-alignment/598091#598091
documentclass[a4paper,landscape]{article}
 
usepackage{graphicx}
usepackage{multirow}
usepackage{amsmath,amssymb}
usepackage[margin=2.5cm]{geometry}
usepackage{array}
 
newcolumntype{P}{>{centeringarraybackslash}p{6em}}
 
begin{document}
    begin{table}[!h]
        caption{Análise de Sensibilidade dos parâmetros $overline{N u}_{L_{p}}$ e $R a_{L_{P}}$, em função das respectivas incertezas de medição de influência.}
        %
        begin{tabular}{c*{4}{|P}|c*{1}{|P}|c}
        hline
        multirow{2}{*}{
            begin{tabular}[c]{@{}c@{}}Variação Percentual da Incerteza de Mediçãoend{tabular}} & 
            multicolumn{5}{c|}{
                Variação Percentual do $delta_{overline{N u}_{L_{p}}}$, conforme variações nas incertezas:
            }
&
multicolumn{2}{c}{
    parbox{16em}{
        centeringsmallskip
        Variação Percentual do $delta_{R a_{L_{p}}}$,
        conforme variações nas incertezas:
        smallskip
    }
}
 cline{2-8} 
                                                                                                               & $delta _{C_{t}}$ & $delta _{m_{P}}$ & $delta _{L_{P}}$ & $delta _{epsilon}$ & $delta _{overline{T}_{text {inicial}}} = delta _{overline{T}_{text {final}}} = delta _{overline{T}_{infty}}$ & $delta _{L_{P}}$ & $delta _{overline{T}_{text {inicial}}} = delta _{overline{T}_{text {final}}} = delta _{overline{T}_{infty}}$  hline
        -90%                                                                                                  & -0,1058%         & -0,0797%         & -0,0029%         & -88,0945%           & -0,0204%                                                                                                             & -0,0879%         & -89,1567%                                                                                                             hline
        -75%                                                                                                  & -0,1002%         & -0,0755%         & -0,0028%         & -74,2216%           & -0,0193%                                                                                                             & -0,0833%         & -74,6693%                                                                                                             hline
        -50%                                                                                                  & -0,0801%         & -0,0604%         & -0,0022%         & -49,6848%           & -0,0154%                                                                                                             & -0,0666%         & -49,8670%                                                                                                             hline
        -25%                                                                                                  & -0,0467%         & -0,0352%         & -0,0013%         & -24,8771%           & -0,0090%                                                                                                             & -0,0388%         & -24,9482%                                                                                                             hline
        +25%                                                                                                  & 0,0601%          & 0,0453%          & 0,0017%          & 24,9051%            & 0,0116%                                                                                                              & +0,0499%         & +24,9600%                                                                                                             hline
        +50%                                                                                                  & 0,1334%          & 0,1006%          & 0,0037%          & 49,8242%            & 0,0257%                                                                                                              & +0,1109%         & +49,9260%                                                                                                             hline
        +75%                                                                                                  & 0,2200%          & 0,1659%          & 0,0061%          & 74,7514%            & 0,0424%                                                                                                              & +0,1829%         & +74,8953%                                                                                                             hline
        +90%                                                                                                  & 0,2784%          & 0,2099%          & 0,0077%          & 89,7102%            & 0,0537%                                                                                                              & +0,2314%         & +89,8780%                                                                                                             hline
        +100%                                                                                                 & 0,3199%          & 0,2412%          & 0,0088%          & 99,6835%            & 0,0617%                                                                                                              & +0,2660%         & +99,8668%                                                                                                             hline
        end{tabular}
        label{tab:sensib}
        %legend{Fonte: Próprio Autor.}
    end{table}
end{document}