I don't have any calculations for large XOOPS sites but I would say make sure persistent connections is off.
In mainfile.php the line is:
define('XOOPS_DB_PCONNECT', 0);

0 = off, 1 = on

I have yet to see lingering connections from XOOPS even with xoopsgallery.

Could help to install a stand alone phpmyadmin and have a look. Without root access you won't see all the juicy info but you'll see your own.

I have run large and busy PHP/MySQL sites and never saw it crash. But I wasn't sharing the server with unkown users and sites. The host server might have issues.

Wanted to confirm that DonXoop is correct. I finally heard back from my provider and it turns out someone else's php/mysql site was using persistent connections and it exhausted the db connections. I've gone back and made sure all my XOOPS sites have persistent connections turned off. But, as mentioned, on a shared server you're at the mercy of other users, the safest way is still to host it yourself.

Jason