You'll have to be more specific. Where did you read this? What OS version are you referring to?
Group management has changed over time in Mac OS X, as have the other capabilities and constraints of account management. No one answer applies to all OS versions.
As a test, I just added my user acct to some ad hoc groups I created, running under OS X 10.8.2, and there are now 18 groups that acct is a member of. So for this OS version, there is not a limit of 16. There may be a higher hard limit, or simply a pracitical limit at some level.
Whether there are hard limits in earlier OS versions, you'd have to tell us what version you need the info for. I have several at my disposal, but I'm not going to do an exhaustive test just for fun.
As to order of groups, I don't think you have any control over that.
Exactly what problem are you trying to solve that leads you to use groups, and ordered groups in particular? Maybe there's another way to solve that problem, if you told us what that is.
http://www.perlmonks.org/index.pl?node_id=542341
.. You want to do X, and you think Y is the best way of doing it.
.. Instead of asking about X, you ask about Y.
Since I first started using Unix, I have seen any number of attempts that use groups to solve a particular problem. These attempted solutions almost never work out in practice, usually due to scalability issues.