It has alot to do with your predispostion to low sperm to begin with. I have friend that has never done any drugs and is have a hard time having kids.
I am going through this right now. currently have a 3 year daughter and wife wants another. I was cycling different mixes of test, dbol, deca, and eq for an extended period of time without a break (over a year).
my sperm count was at only 10 million, normal is 60 to 200 million.
Have been off since september, Urologist prescribed 50 mg clomid for 25 day to get LH Levels back to normal and induce sperm production again.