Manual:$wgAccountCreationThrottle/en

Details
Limit to the number of accounts which may be created within a 24-hour period from a single IP address (whether by unregistered or registered user), 0 to disable. Account creations from users with the  right (or in the deprecated   group) aren't counted.

It's set to 6 on Wikimedia Foundation wikis.

Requires memcached.