I want to know how often do crashes happen to bikers?
I think it depends on how you ride (and what you ride, and where, and when).
For example, I was talking with a young man recently, who owns a racing bike. We were talking about where we ride and I told him that I'm not comfortable on a road which has two lanes in each direction and a speed limit above 50kph/30mph. He said, "That's ridiculous." I remembered that he'd said earlier that he'd broken bones by slipping on a wet man-hole cover when tearing round a round-about, and replied, "Perhaps, but I've never broken any bones."
I guess I ride with caution: not always slowly, necessarily, but definitely varying according to how I feel (and what my past and present experience tells me) about the road conditions. I'm commuting, not racing.
I fell off three times in the first week of riding my current bike: twice, from slipping on ice, once from not unclipping in time when stopping; each of these times I was travelling at about 5kph/3mph or less, and so not hurt (not even scratched, wearing a winter coat, just one mild bruise).
Going downhill for example now, I'm concious that there might be sand or gravel on the road, that the pavement may be cracked in places (by frost, this is in Toronto). I don't ride slowly, but I do try slowly enough, to stay in control: compare that with this answer.
When I posted this question most people answered that I needn't expect to fall at all again.
The reason I'm not comfortable on a multi-lane road is that, on such a road once, during rush hour, a motorist hit me from behind by trying to undertake someone else while I was in the curb-side lane. Well, that hurt (me and my bike). Luckily it didn't stop me riding, but it did stop me riding on that particular stretch of road, and made me more wary of being hit from behind. I don't expect it will happen again: partly because I avoid that kind of road now, partly because I'm more cautious, sometimes stopping by the side of the road to let a rush of cars overtake instead of pulling out into traffic and just hoping that someone will slow down for me.
Also how do you recover from them, both physically and mentally?
Is "how to recover physically" a medical question? It depends on the injury: see this Q&A for some details.