How often do retrotransposons jump in human cells?
For just rough order-of-magnitude, try this reference. Their experimental design seems to maximize transpositions so it should be a reasonable upper bound.

There is a lot of variability, but some retrotransposons jumped in ~1000 cells per million, others only a few per million such that they were difficult to detect, after about 1 month in HeLa cells.

The same reference gives an estimate of 10,000 retrotransposon elements in a mammalian genome, though, again, their transposition frequencies vary considerably.