2 Keypads

@ mtrclist short answer is yes. Heck, go crazy, do three or four! Kidding of course. If you have any other questions, use the new help center or call SS and speak with sales.  Great system and great company; yes, there are some issues but a customer of 9+ years and very happy as a customer. Good luck.

Yes, I have one by the front door and one in my master bedroom. They work just fine but be aware that during system updates you need to update each keypad one at a time.

Two is very common (I have 2).  More would be just as possible, up to the maximum number of devices defined to the system.

I have 4, and yes, it's a PITA when doing firmware updates, particularly because the updates kill all the batteries and I find that I generally have to replace all 16 batteries prior to an update.  That said, I couldn't live without the 4 keypads, so it's a tradeoff :)