Subject says it all. =] I have heard that it is possible to get all the civs in the game at once, but I have no idea how to get more (or for that matter less) than the default (7 or 8 I think).
The number of opponents depends on your land size you select, if you go for massive then you can select them all. (nb you may get a more detail around this in the help forum)
You can use the editor to make this happen, the editor contains the settings for the maximum number of civs on each map size... (I believe its even possible to set it for 16 civs on the tiny map).