Hi Ricardo,
Cisco MCS 7845 (All supported models)
Maximum Users per Server 7500
Cisco MCS 7835 (All supported models)
Maximum Users per Server 2500
Cisco MCS 7825 (All supported models)
Maximum Users per Server 1000
Cisco MCS 7815 or MCS 7816 (All supported models)3
Maximum Users per Server 300
From this excellent SRND 6.x Guide;
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/srnd/6x/callpros.html#wp1043871
Hope this helps!
Rob