I think more than you can do with good form is too many.

I often see guys at the gym doing sets of 30 pushups with their backs arched so their stomachs are nearly touching the ground, their arms straight, going down 2 cms at a time with their shoulders only - Very, very bad and painful to watch.

Give me 5 good ones with a firm stomach holding everything in line properly, going all the way down and all the way up any day over 30 crap ones!

Seriously as a girl it was hard for me to learn to do push ups and it took a long time. Luckily I was taught to do them with correct form and now I can do as many as most guys.

Taking a little longer, doing fewer reps, to get the movement right to begin with paid off. If you can't do large numbers properly on your toes you can always add some on your knees on a soft mat or some chest press.

One thing to be careful of is what you do with back pain. I was told to cut down on pushups a bit when I started getting back pain. Now I make sure i do a lot of back strengthening work to balance it.