How big is the chance to get pregnant if my husband has low testosterone?

Depends . . .: Low testosterone doesn't always imply low sperm count although latter does make it more difficult to impregnate one's partner. So if fertility is an issue along w/low t, 1st get a sperm count. If low, consider using (way) off-label hcg, Clomiphene or tamoxifen to increase both t & sperm. Whatever you do, don't fix low t w/t which will only render you really & truly infertile if you weren't already.