The Vattendance System enables 40 simultaneous callers by default.

However the amount of callers who can dial in without service degradation depends on a few factors

The bandwidth of your internet connection
Each concurrent caller uses approximately 100kbps of upstream bandwidth by default. For PBX's that are using an ADSL connection, it will automatically adjust to using a lower bandwidth audio connection. In this case each caller would use 45kbps.

How the internet connection is being used?
If you are providing WiFi access to members of the congregation then this will also eat into your bandwidth. It would be wise implement a router or access point that can prioritize VoIP traffic over normal internet traffic.

The quality of your router
Using a consumer grade router can result in a degradation of the VoIP service and a limit of how many VoIP connections can be established at one time. Routers provided free of charge by your internet service provider often have this kind of limitation.