This depends on your onsite PBX and its supported codecs. If your PBX supports G.729, you only require 0.03 Mbps including overheads. Therefore, if you have
a reliable connection of even 8 Mbps, you could have about 260 simultaneous calls connected. However, quality of calls will highly depend on the reliability
of your internet connection, so it does not have to be very fast but it must be reliable and stable.