What is the Limit for the Number of NTP Clients?

The limit actually depends on several factors, like speed of the main processor and network bandwidth, but the limit is quite high. Terje Mathisen once presented a calculation:

• 2 packets/256 seconds * 500 K machines -> 4 K packets/second (half in each direction).
• Packet size is close to minimum, definitely less than 128 bytes even with cryptographic authentication:
• 4 K * 128 -> 512 KB/s.
• So, as long as you had a dedicated 100 Mbit/s full duplex leg from the central switch for each server, it should see average networks load of maximim 2-3%.