It's been a long time since I did pushups to failure, but at one point I could crank out about 75 or more and could do at least 5 one-arm pushups on each side. One-armed ones are different though, and if you want to be able to do them, I think it would be better to just practice them rather than waiting till you can do a certain number of 2-armed ones before trying.
Either way, good luck!