You cannot limit bandwidth usage and number of connections to a site; there is no
Bandwidth Limiting option in Websites & Domains tab (in the Advanced Operations group) even if
mod_bw is installed.
Bandwidth limitation is currently not available if
Nginx is enabled. The feature request number 1661520 has been already submitted regarding the matter.
If you disable
Tools & Settings > Services management > stop 'Reverse Proxy Server (nginx)'
then the 'Bandwidth Limiting' option will be available under the Websites & Domains in the Advanced Operations group.