NTP is working with UDP packets.
The size of one packet is less than 128 Bytes, approx. 100 Bytes.
The traffic is defined from how often the clients are requesting the time from a server.
NTP clocks are usually configured to request the time from server every 10s.
This means the following traffic:
1 NTP request packet and 1 NTP answer packet, every 10s, each with approx. 100 Bytes.
==> 200 Byte per 10s or in average 20 Bytes / sec or 160 Bits / sec.
So for e.g. 1000 clocks, it is in average approx. 160 kBit / sec.
In master clocks you define with the poll interval, how often the time is requested from a server. The min poll interval value is usually 3, means every 8s (2 * 2 * 2 = 8).
So the traffic is almost the same as for slave clocks.
Mainly the bandwidth for NTP is minor, compared with other network traffic.