another point to consider would be with what will happen to your cats if something happens to you.
My mums 62, and she's said that she'll only get more cats (after her present moggies have gone over the rainbow bridge, presently they are 15) if I'll promis her to adopt them if she can`t take care of them anymore. She wouldn't want more than two cats, also for this reason. She wouldn't want an older cat of hers go to strangers or to a shelter, which I can understand.
So I'm be prepared to adoped her cats to save them from the shelter and give her a rest of mind on this issue.
If she would have more than two cats I probably couldn't keep this promise.
Of course there's always the off chance that a stray somehow ends up permanently with her
.. which would also be OK. But the limit of planned cats is set.