How many Type II restrictions enzymes are currently available (commercially)
The Restriction Enzymes section at Biocompare is likely not comprehensive, but short of contacting every single molecular biology company and merging all their inventories into a master list, it's probably the best you can do on short notice. They currently list 289 uniquely-named enzymes, but I'm not enough of an expert to say how many are similar in activity/recognition sequence, or whether they might also be known by other names not currently listed. For your purposes, however, it'd probably be enough to tell your students that there are on the order of 300 common commercially-available enzymes. Hope this helps.