A question regarding the derivation of the equation for the Planck black body radiation distribution

There is something that does not make sense to me regarding a derivation that I am reading of the Planck distribution for black body radiation emitted from a cavity. It treats the photons as particles, uses the 3D wave equation (from Maxwell’s equation reasonable as photons are exclamations in the quantum field) applies a boundary condition of zero on the edges of the cavity treated as a box, all of which I am fine with. Photons are bosons, so that for any given wave vector $(px, py, pz)$ in the discrete space of momentum states there can be many photons,and each wave vector identifies one state; again fine with all this. What follows after this is where I have a problem. The derivation calculates the partition function as follows. It picks a single state $(px,py,pz)$, identified by one wave vector and "sums the states" corresponding to different numbers of photons in that single state as follows.

$$Z=sum_{n=1}^{infty}ne^{-nhnu}$$

Where $nu$ is the energy associated with that wave vector $(px,py,pz)$. To explain my problem, let me consider just two terms in this sum, with m > n, as shown below.

$$Z=…+ne^{-nhnu}+me^{-mhnu}+….$$

So the first "n" term corresponds to the case where there are n photons all in the considered momentum state $(px,py,pz)$. It says that there are n particles in this state $(px,py,pz)$. The $m$ term similarly says that there are m photons in the state $(px,py,pz)$ – a contradiction, because as as we just saw, there are $n$ terms in this state!

Where am I going wrong? What am I misunderstanding?