I didn't have any TSTO friends for about a month. Then I added 25 and it ballooned to 100 quickly which caused my game to crash like crazy. I deleted them all and slowly added back in the ones I had really liked. I keep getting invites so it has quickly exploded back to 90. I don't like having that many because I feel guilty if I don't visit each one every day so after the holidays I will go back down.
Right now the plan is around 50 and I've identified 40 that I have to keep. These are people who are active on here or who have towns that I love. I actually plan on keeping a coupe of farmers because I like their towns. I also plan on keeping a couple of lower level players because they have done great things with their towns already and I'm excited for them.
So the driving force for me is interaction on here, I like having friends who are active on this forum and who I feel like I'm getting "to know". Second to that would be the aestetics of their towns.