There is no limit to the number of A/B tests you can run simultaneously.
Would you like to run an A/B test on 10 target groups at the same time? You can!
Just keep in mind that you can send one newsletter per day to your customers. If you already perform an A/B test on a certain target group that day, then the 2nd A/B test on the same target group on the same day will fail.
So choose a different day to test, or set up a simultaneous test for a different audience.