How many concurrent VPN IPsec tunnels can be open on a RB750. What is the technical limit, and/or is there a license limit?

The license level page (http://wiki.mikrotik.com/wiki/License_levels) doesn't mention IPsec tunnels. The technical limit is going to heavily depend on how much traffic you're pushing through the tunnels - keeping a lot of virtually idle SAs around isn't a lot of work, but encrypting/decrypting 10mbps of traffic is.

from my expirence RB750 can handle up to 9-10Mbps half duplex ;) it's quite small... :)

That is good news! So that means the limit is basically how much the CPU can handle. Since we are pushing through very little data, chances are that we can handle dozens of IPSec VPN tunnels? That certainly sounds good