The key word is always TIME. Anyone who says otherwise, might as well not know their arse from their elbow. I have been in leagues that are so complicated that they require an actual rulebook to leagues that simply the rules could be fit on one sheet of paper. But one thing remains consistent and that is time. Now time does not determine your success in the league alone but at the very least you know, as a player, that you put your hard active effort into it. Now that does not mean turn it into the obsession, especially for the beginner.
There is a delicate balance that needs to be achieved and you will know that balance after playing a full year of leagues. If you feel like you can handle a few more leagues then go for it but if it feels like you are way over your head, then pull back immediately. Nothing is worse than trying to bite off more than you can chew. I have done it many times myself and it is never a good feeling. The worst thing is you often let more than yourself down, you also let down the people in those leagues. Let's face it. We all crave a high level of competition in some sense (or at least some level) and that is why most of us join these leagues in the first place.
I was in at least 5 or 6 different kinds of leagues last year alone. However I cut my amount of leagues down to 10. If I told you what the previous two years were, you would fall over. I will give you a hint. It was somewhere in the 30-40 range. That is way too many, even for a "expert". The problem is when you get all these invites and then you do not want to let people down who took the time to invite you. Sometimes it is okay to say no and always choose the leagues you want to go into and not the other way around.
So keep in mind that I encourage you to join as many leagues as you can at first to get your feet wet. However, if you find yourself just drowning in leagues or league types, the best thing to do is cut back until you can handle it. There are ways to gauge it such as the enjoyment factor. How much did you enjoy first, the sum of the leagues and second, each individual league? It seems like a nasty question with 99 parts but it really is not. Take as much time as you need to over the summer thinking of the time constraint first and foremost. Almost 90 percent of people that go inactive in leagues go inactive because of time issues. Sure things happen in life that take precedent over fantasy sports but just keep it in mind.