Re:

Is a separate AS Number required for an Internet Xchange point network.

almost all ixen are at layer two. hence they have no ip addresses, let alone layer three routing. of course, the ix provider may also choose to provide services on the side, such as ntp, measurement, ... those are servers and run at layer three+. hence they are routed. like other routed networks, if they are multi- homed, they tend to have their own asn. randy