Most likely there won't be a lot of them- maybe as few as five and probably not many more than ten or fifteen total. These are loud enough to be easily heard in most roasters. The variables are considerable. There is no specific rule. These are not as loud as first crack, more difficult to hear, but they happen a lot more frequently and in greater numbers than first cracks.
Second crack takes place over a longer period of time as well.
They start slow and then build to a point where there may be one or two a second for a bit and then decrease in frequency. In some blends active second can happen two or more times during a roast. I will remind you that particularly with second crack, the time, rate, and duration can vary dramatically based on the variables I mentioned earlier. Second crack is very important. Many home roasters use this as THE indicator when roasting.