21 / (1.1m / 2) = 26,190.
21 is the number of US servers for GW2 as of today.
1.1m is the rumored/reported/whatever number of pre-orders for GW2 right now (or something).
2 because EU will have its own servers.
26,190 people per server, assuming everyone splits evenly (0.0 chance of course).
26,190 people all trying to log into one server is going to go… poorly. Anet is not CCP and GW2 has ground which is like, totally different from space, yo. And yes, on day one everyone is going to attempt to hit the server. (If not way more considering that 1.1m number is far from final)
More servers is the easy answer, except that in addition to likely having to close said servers later, Anet has made it pretty clear that transferring is tricky due to how permanent a server choice is, and how it might impact WvW standing.
What would Jesus do?