Brandon Sanderson's Third Law of Magic states "expand what you already have before you add something new". This refers, at its heart, to the complexity of magic systems, but the principle drips down into many areas.
A novel can only support a finite amount of elements. There isn't a specific number. It simply has bounds, somewhere. Readers have further limitations to their patience. If you throw a million different magic systems, five million characters, ten million important settings, and twenty million subplots into the same book, you'll have two major problems: the book will be dreadfully long and ninety-nine percent of readers will get confused and frustrated by chapter eight (or six or twenty).
Making a few attributes of your world-building elaborate will not only cut reader anxiety to shreds, it leads to wittier, more satisfying prose as well. If there are half a dozen magic systems you want to incorporate into a novella, you'll probably want to combine and subtract powers until you're down to one or two well-developed systems. Brandon suggests doing so by finding the commonalities among your magic systems (or whatever it is you're combining) and linking from there. If two systems heavily involve nature, merge them into one system with a strong controlling purpose.
Don't forget that everything you write affects everything you've written and shall write in the future. Magic sculpts economies, politics, medicine, etc. You can start at any place in your world-building, yet your first decisions cascade everywhere else. If you want a very war-intensive magic system, the culture of the people who use it must be influenced by it, most likely becoming violent and fierce. The people may barter in the blood they stripped from their enemies. They may worship a god who uses a blacksmith's hammer. Some type of tree, the best for a light-weight shield, may be sacred. A few seeds planted into your world will spring up into a full-fledged harvest. The soil can't support too many plants. They will choke each other out and deplete all the available nourishment. I won't even start analyzing metaphorical crop rotation...
The basic lesson here is to not throw a pinch of every spice on your rack into your stew just because you want to give to more flavor. Everything about it is bound to be terrible. Choose a few key spices, chosen with the foods in your stew in mind, and drop them in to compliment the dish. If something doesn't taste quite right, don't toss something new in, tweak the ingredients you already have.