Online on the ps2 there is about 30 servers running at every time of the day, but usually only about 10 of them are really active since alot of them are just empty servers with nobody in them.

There is always like 2 or 3 official servers that are always full with 20 or more people in them at every time. And there's 3 servers that only deal with hero assault mode too that are usually always full.

Then a few more servers are popular clan servers like JSA or MERC clans that usually have team damage on and sometimes you can't play as special units on them or you might need much more points than usually to play as a special unit, like 50 points to play as a dark trooper instead of the usual amount which is like 12 I think.

EDIT: whoops didn't know that you were talking about Xbox live but if you have a ps2 then online should be pretty active at most parts of the day.