Are there asymptotic upper and lower bounds on the "total" number of subspaces of $\mathbf{F}^n_q$? $q$ is fixed and $n$ increases
The following two answers are helpful but dont answer the question.
https://mathoverflow.net/questions/89934/sum-of-gaussian-binomial-coefficients
When $q=2$, these numbers are tabulated at A006116 where many references and links are given. Maybe some of those lead to other values of $q$ and more general questions. It says the number is asymptotic to $c2^{n^2/4}$, where $c = {\rm EllipticTheta}[3,0,1/2] / {\rm QPochhammer}[1/2,1/2] = 7.3719688014613\dots$ if $n$ is even and $c = {\rm EllipticTheta}[2,0,1/2] / {\rm QPochhammer}[1/2,1/2] = 7.3719494907662\dots$ if $n$ is odd.
When $q=3$, the numbers are tabulated at A006117, and again there's an asymptotic formula of the form $c3^{n^2/4}$, where $c$ is given in terms of EllipticTheta and QPochhammer. I'm sure there's more where those came from.