Yes, in a conventional home oven, multiple cakes in the oven will affect baking time, and may affect the general outcome of all of the cakes, as well. Personally, I don't recommend it.
I, too, bake one cake at a time unless two or more layers will fit on one rack in the middle of the oven--and, only if there is room to circulate air between all of those layers and the oven walls.
If you put cakes on multiple levels in a conventional oven, the cakes on the bottom level may not brown on the top and may overcook on the bottoms. Generally, the cakes on the upper rack will do the best, but the baking times will be hard to calculate and you may have to open the oven too often.
If you have a commercial oven and/or a convection oven, you can bake as many layers at one time as you can fit in the oven box.
Rae
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