On my laptop I can't get more than about 40 matrices in an array before it bombs out.
DX9 hardware supports 256 vertex shader constants.
Assuming a 4x3 matrix for each bone transform, that should get you at best 256/3 = 85 matrices with one constant left unused.
That's not very realistic though. At a minimum you want 4 constants for the World-View-Projection transform, and probably a few more than that for other stuff.
Based on that, 80 matrices is a reasonable goal, and 70 should fit easily.
Thanks for the info.