Extract by expression - N first

Use this expression:

array_foreach ( -- iterate over all entries for each value of the series created in the next line
        generate_series ( 0 , 9 , 1) , -- generates a series from 0 to 9 with steps of 1 to get the first 10 values
        array_get (  -- get the n-th entry (0 for the first) of the sorted array from the next line
            array_sort( -- sort the array created in the next line
                array_agg ( "value"), -- aggregate all values from the field "values" to an array
                false ) , -- override default sorting order (ascending=true) to get a descending order 
        @element -- get the n-th entry of the sorted array, whereas n is the current value of the series (0 to 9)
        )
    )

See screenshot: I have an attribute "value" with random values from 1 to 2000. Output of the expression in my case is: [2000,1998,1996,1978,1978,1976,1961,1958,1956,1955]. The output is obviously an array, so depending on what you neet it for, you must convert it.

As MrXsquared suggested, you can use array_contains(array,"value") (replace array with the expression from above) to generate a boolean value to your attribute table with true for the n first (10 highest) values. Without that, the output is an array cotaining the 10 highest values.

If you wish to use "extract by expression" you can use this expression:

 "yournumericalfield" >= array_get(array_sort(array_agg("yournumericalfield"),false),9)

Replace "yournumericalfield" with the fieldname you wish to use as reference and replace 9 at the end of this expression with the highest wanted value. In this example it will extract the features with the 10 highest values.

If you would want to extract the 10 lowest values, you could replace false with true.

In case your field is not numerical, but contains numbers, you can add to_real() to the expression.

You can try the QGIS processing algorithm Execute SQL (documentation) with :

SELECT * FROM input1 ORDER BY YOUR_FIELD desc LIMIT 10

• SELECT * : select all data from your layer
• FROM input1 : according to the documentation, input1 refers to the first input datasource, so your layer
• ORDER BY YOUR_FIELD : sort your data result according to YOUR_FIELD, a field of your layer. You can add the word DESC at the end for reverse order
• LIMIT 10 : take only the 10 first records

This processing will create a new layer with the extracted records.