Jackson will probably be 88, then, or 18. I prefer 80s for receivers but whatever, it's not my decision (Having numbers for positions is one of the things I prefer in the NFL over college... one of many things, really). The new receiver numbers are in the teens or 80s... it used to be you had to fill up your 80s before players could use numbers in the teens, Keyshawn being the notable exception who asked the league if he could be 19.
The numbers the guys wear at the first rookie camp are usually the ones they end of wearing, but not always. Warren wore 92 with the Patriots in camp and preseason until Ted Washington was traded for, and Washington wanted to wear 92 as he always has (though I vaguely remember him wearing 72 or 75 with the Bills, but I might be mistaken) so Warren deferred to him and wore 94, which he continues to wear now.