Skype is peer to peer. That this THE clever invention that put them ahead of all competing services when they launched. They can scale without any significant additional bandwidth or server costs.
This is true only for Skype to Skype connections. Calls that have to reach The PSTN[1] need to be routed through telephony servers somehow, exposing the possibility to cause some stress on Skype's servers as FrojoS mentions.
[1] http://en.wikipedia.org/wiki/Public_switched_telephone_netwo...