As far as I can see currently the most Thunderbolt Gigabit Ethernet adaptors you are going to be able to connect is two, to an iMac, this along with the built-in Gigabit Ethernet port would give you a total of 3 x 1000Mbps if you trunk them together.
If you need more than this then you should look at PCI-E 10Gbps cards. Either directly in a Mac Pro, or via a Thunderbolt to PCI-E expansion chassis.
This approach is not normally needed. You would use a Thunderbolt to FDDI adaptor see http://www.attotech.com/products/category.php?id=15&catid=16
You can also get a Thunderbolt to 10Gbps card however both are a cheat, it looks like a Thunderbolt to PCI-E expansion box and then a standard PCI-E card.
This is a bit more genuine see http://www.promise.com/media_bank/Download%20Bank/Datasheet/SANLink_Series_DS_v1 .02.pdf although on paper it is not as fast.