Interesting stuff. The fact that it was illegal makes it tough to get a hard number, so you have to rely on anecdotal and indirect evidence rather than commercial and other records, and you have to make a lot of inferences.
There would also be all kinds of problems with bills of sale and other methods of proving title. Under the law any owner, not just the slave importer, had to prove the slave in question had been brought into the country at least five years before the prosecution. Sounds like for many planters it would have been way too much trouble and risk.