As much as I would love to take credit for being right, you may be experiencing the disconnect /follow bug as discussed in other threads.

If performance is ok for you in IF or other clustered areas then box as many as you can. I would imagine that 3 would be playable at the same time with a high enough graphics card, enough memory and a dual core or better. Your version of playable and mine might be different though.

Check the other threads about the /follow bug and (hopefully temporary) workarounds.