I think it also depends on how long of novel you're writing. A 7 book series like Harry Potter is going to need more subplots than a novel that is a stand-alone at 40,000 words.
I, myself, have had to delete subplots for my novel that I'm writing. I try to keep it concise and clear. End each book with a completed mission, which is the subplot and continue with the 'save the world, defeat the bad guy' plot.
In a time-traveler series I'm working on the goal is to get back to the future, but each book is a stand-alone adventure back in time. The first book the kids go back in time, have to try and save a girl from being convicted of witchcraft, meanwhile there is a demon who is cursed to wolf-form and his demon brother is trying to help him break the curse with the help of a witch--then they meet the time-traveler kids and go after them. And the Indian girl who is one of the main characters tries to warn King Phillip of what will happen to the Indians in the future.
So that's like 3 subplots, but I think it reads clearly. I think the ultimate thing to ask oneself when writing subplots is, do all these dots connect and form a perfect circle? Can I write the same story without this going on? Do I need this character? Do I need this drama. If you need them, you need them, that's all there is to it.
I'm pondering about slicing out some characters right now, but I'm still debating.