You can have as many as you want, but I wouldn't put more then 20. And some advice, use phrases. Like if you're site is about red apples, you would want to put your keywords something like this:
"red apples, food, fruit, healthy, etc."
I could get really techincal and explain why this works better then just putting "red, apples", but think about it. If it doesn't click, e-mail me and I'll explain it.
Hope this helped. ~Steve
A broken clock is correct twice per day.
A roomful of monkeys on typwriters will eventually write the great american novel.
If you gave an infinate number of rednecks, an infinate number of shotguns to shoot at an infinate number of street signs, they'd eventually recreate all of Shakespears works in braile.