Forums take a lot of dedication. I think a lot of forum owners have too many forums, and they should focus on one forum. I do have two forums, but I definitely focus on one forum more than the other.
Currently, I am managing 8 forums. Since I am doing this as a full time job, and I get paid for managing forums, I am handling the forum managing jobs well. No complains have been lodged so far. Once I was running 3 forums of my own and I was also running them well even though only one of them made actual money. Whether you can manage multiple forums or not depends on your level of skills, available time, and also interest.
I agree with you here; I think if you want a community to grow and be successful, you should put all your focus into it and maybe look at creating another forum later in the future. I do think having multiple forums/communities is great, but focus on one and make it successful and then move on. But don't overload yourself, as this is where failure occurs, if that makes sense
It depends on the person. There are people who are capable of managing more than one forum, but not everyone can. You need to be able to focus on each one and make sure none of them fall behind. If you do get overwhelmed, your best bet would be to see if another person could take over or merge your existing communities together.