How many web agents does my site require?
It all depends on how many visitors your site receives daily.
On average 1 chat agent for each 100 daily visitors should be sufficient.
So if your site serves 200 unique visitors daily it is recommended to have 2 chat agents assuming the live chat button is installed in visible location on your website.
If you are having troubles staffing your chat, please consider outsourcing to a live chat BPO.