Required amount of memory is depends on total number of monitored hosts.
You could use this formula for calculations:
total_number_of_hosts * 208 * 3
- 3 is a number of counters (data counter, current speed counter, smoothed speed counter).
- 208 is a total size of traffic countring structure for 1.1.3 version.
- /16, 65535 hosts – 40 mb of RAM
- /8, 16 millions of hosts – 10GB ram
But please keep in mind! We need to iterate across all counters with single CPU core in ~1 second. Then, for big number of hosts you need to have fast CPU core.