you can have as many anemones as you would like in a tank. the problem with anemones are:

they move around a lot until they find a comfortable stop.

they will sting/kill fish & corals that get too close

they split every every 1-6 months so you could have more anemones than you would like. I had an a rose not split for 6 months and then one night split into 4.

if you have a male and female anemone they can spawn under the right conditions.