About how many aborted takeoffs are there each year in commercial aviation?
As off today (March 21st), Aviation Herald lists 26 rejected take-offs, in 2018. 

It's 110 days since the start of the year. So that's 110/365ths, or 0.31 of the year. So, the whole year will have 26/0.31, or about 84 rejected or aborted take-offs.

This assumes, all are reported in Aviation Herald, which is probably not true, so the real average number is likely to be higher. But that's a rough starting point, and probably the best estimate you will get.