Hello there,
There are no hardcode limit to the no of gatways on a gatekeeper. However the main limitation will be the amount of existing main memory used to keep the gateways and endpoint info which are dependent on the name and e.164 address.
If you are interested in how many E.164 address can be registered on a Gatekeeper, then a target of 100 gateways per gatekeeper would be the way to go.