I know that this question is answered in https://www.pool.ntp.org/join.html . However, I think this information is outdated. In that page it is said that most servers get about 5-15 NTP packets/sec, with spikes a couple of times a day of 60-120 packets/sec. This is roughly equivalent to 10-15Kbps with spikes of 50-120Kbps. In plain terms, you probably need at least 384-512Kbit bandwidth (in and out-going).
I have an NTP server in Italy with IPv6 connectivity. This server is advertised in the NTP pool project for more than a year. And it gets roughly 1 IPv6 NTP request per 5 seconds, or 0.2 packets/sec. So, the traffic is negligible.
Recently, I opened the same server to IPv4 network and advertised it in the NTP pool project for only a few days. The average packet request rate per second is now around 90-120 packets/sec. Together with the responses, it makes up a bandwidth of 11-15 KBytes/sec roughly. This translates to more than 100Kbps.
I cannot decide if this is normal or a coincidence (a “spike”). Does the documentation need an update?