Alexander merged into Shiva not that long ago so the server's pretty active. The lowest I see on at once is 2500, but it's usually around 3500-4000. There are always people shouting in Jeuno for pick up groups, abyssea parties, etc. I came back myself not long ago and was concerned about being able to do anything without a linkshell, but it's a much more casual-friendly game these days, and I was able to jump back in without any significant problems. The main exception would be for the older missions/quests; you can solo a lot now, but for the stuff you can't, finding a group can be difficult. Traditional parties for lower level jobs can also be difficult to find, but there are many "alternative" leveling methods now and soloing is also much more feasible than before.