Hi Kurl75,
You may have realised by now, that there are 3 main ways to obtain expansions:
1) Research/map conquest, followed by coin/diamond payment
2) Purchase with medals
3) Diamond expansions (does not require research or conquest)
However, there are also limits to how many expansions you can get from each method. The limit from research/map conquest is, of course, the number of technology and provinces that gives expansions. As for medal expansions and diamond expansions, the limit is 2 expansions per age per payment type. That is to say, when you're in Bronze Age, you can only buy 2 expansions with medals, and 2 diamond expansions (making it a total of 4). You cannot buy any more, even if you had extra medals or diamonds. As a result, all players can only get a total of 24 expansions through purchase with medals and diamonds (without research/conquest).
All the above information was obtained from Pendragon's FAQ
(click me), have a read if you're still unclear. From the information given, it seems that the maximum number of expansion that a colonial age player can have, assuming he gets everything, is 62.