Generators of $GL_n(\Bbb Z)$ and $GL_n(\Bbb Z_p)$

Solution 1:

In 1932/3 B.H. Neumann found a presentation for the automorphism group of the free group of rank $n$ on two generators for $n \ge 4$ and 3 generators for $n=3$. These were based on earlier presentations on 4 generators due to Nielsen. A single additional relation gives a presentation of the quotient ${\rm GL}_n({\mathbb Z})$. You can find these results in the book on Combinatorial group Theory by Magnus, Karass and Solitar.

So, for ${\rm GL}_n({\mathbb Z})$, the answer is 2 for $n \ge 4$ and at most 3 for $n=3$. It is well-known to be 2 for $n=2$. It seems plausible that it is also 2 for $n=3$, and I am sure this must be known. I would expect similar results to hold for ${\rm GL}_n({\mathbb Z_p})$.

You could try asking this on MathOverflow.