is it correct that for a nbit LFSR (galois algorithm in particular) the maximum number of unique 32byte keys can only be 255?
An $n-$ bit primitive LFSR generates the $2^n-1$ unique nonzero $n$ tuples exactly once in a cycle.

Take $n=8$ and your questions all have the answer yes, except the all zero seed will not give any tuple other than the all zero tuple.

However, such an $n$bit primitive LFSR generates all $k$ tuples, for $k\leq n,$ (provided its initial loaading is nonzero)

$$2^{n-k}$$
times if the $k$tuple is not all zeroes, and
$$2^{n-k}-1$$
times if the $k$ tuple is all zero.

So these $k$ tuples are not unique, repeat at pseudorandom times along the cycle, but you do get the all zero $k$ tuple occur naturaly in the scheme.

Example: $n=4,k=3.$ There should be 2 of each nonzero 3-tuple in
a period while the zero 3 tuple should occur exactly once. The sequence below is a primitive LFSR sequence. I have taken $2^4-1+2=17$ symbols instead of 15, so I can examine the 3-tuples starting within one period from position 1 to position 15. I grouped in 5's to make it easy to see positions

00100.11010.11110.00

001 occurs twice, starting in position 1 and 4

010 occurs twice, starting in position 2 and 8

100 occurs twice, starting in position 3 and 14

etc.

000 occurs once, starting in position 15