People keep talking about how beta is gonna test the servers to see how much they can handle and the possibility of servers crashing after the release of FFXIV. In my opinion SE shouldn't and probably wont have much trouble with server population. This isn't their first MMO. If anything there may be problems with other peoples PC's running it. This don't mean other things will be fixed by then due to new content and how your PC will react to them.
I remember playing 11 on ps2 and they had some problems with WotG(ps2 only). I would talk to certain NPC's and it would freeze up on me. They got it fixed.
As far as what a server can handle. They have been dealing with those for a long time and for the most part know what it can do. Most of them lag issues people experienced in 11 was due to their systems not SEs.