How many layers are there in a lasagna?
There’s no right or wrong answer to that. Other than, “more than one” probably. Even two is pretty skimpy.
So, let’s say three is minimal for argument sake. (But, four or more is better.)
The number of layers probably just boils down to using up all the lasagna noodles and sauce you have, or getting to the top of the pan you have to use. No exact amount. Use what you’ve got.
But, if all you’ve got won’t stretch to make three layers in the size pan you have, perhaps you should use a smaller pan. At least have the three layers.