The FAQ at support.ntp.org says to use at least 4 [1] because math.<p>But it basically boils down to "With at least four upstream servers, one (or more) can be a "falseticker", or just unreachable, and ntpd will have a sufficient number of sources to choose from."<p>I've spent a bunch of time sorting out NTP in recent years, chrony does indeed keep much better time and converge more quickly.<p>I would be careful about the advice around NTP anycasting. While attractive as a solution (one IP!), there are tradeoffs and it would be important to take those into account before going down this path.[2]<p>[1] <a href="https://support.ntp.org/bin/view/Support/SelectingOffsiteNTPServers#Section_5.3.3" rel="nofollow">https://support.ntp.org/bin/view/Support/SelectingOffsiteNTP...</a>.<p>[2] <a href="https://www.rfc-editor.org/rfc/rfc8633.html#page-17" rel="nofollow">https://www.rfc-editor.org/rfc/rfc8633.html#page-17</a>