Johnnantan,
The SNMP Community String allows you to monitor the device. I usually recommend that for an enterprise, the community strings be the same for all devices to allow Read Only access to the devices. You can have many different software platforms monitoring the devices at the same time. I generally do not set Read Write community strings or have completely different strings for RW and ACL's.
The limiting factor is on the monitoring device as to how many devices that it can monitor and how much information it is requesting from the monitored device.
If you want to control who is monitoring the routers/switches/servers with snmp, you can implement ACL's on the community strings to only answer snmp queries from specific devices only if they have the correct string. If you do do this have the access list log failures so you can go back and find out what is scanning the devices.
Remember that snmp qureies do take some processing power so that the more devices that are monitoring a router and with short times between queries will have a performance affect on the router or switch.
Hope that this helps.
