Online poker, how many tables do you play at the same time?

One or two. But I'm guilty of playing up to 6 at times but low micro stakes because mistakes will happen and full concentration is not there for one game. Also I think it's the fear of missing out that makes us play multiple tables at once if its tournaments and if its cash just to make a lot of money in a bit of time.

I keep playing 2 but find if I’m doing well on 1 table I tend to lose concentration in the other

I am finally getting better at multitabling, I used only be able to play one now I can play up to 3 or 4. I do find that I play better with one table though so if I am taking a shot or playing a game that is important to be I try to just play the one if I can.