I believe generally most airlines prefer if you have stuck to 1 max 2 FTO's, for the simple reason that it's much easier for them to check your history and if you have any issues during training in the past.
It might not always be practical to do all your training at one FTO, due to the various programs they offer, that's why I believe 2 would be acceptable.
They might start asking awkward questions, if you have been hopping around between 5 - 6 or more FTO's, wondering if you might have had some issues during training, I believe it will bring up a flag, and you need to be ready to explain why during an interview.