How many decals is too many?

You could donate to others that are interested in them. 

Use as many as you want.  This is especially true if the build is just for your own display.

You did not say if you enter contests.  If you do, and want to win awards, then it may be important to use all or most of the decals.

Some kits include decals for several prototype aircraft, but in that case the decal/painting/marking instructions usually point out which decals go on which aircraft.

I'm of the school that says YOU are the one who has to be happy with it, so if adding all of those stencils is in your opinion, just too much, then stop. 

I don't always use every decal on my builds. Just the ones I want.

It can get boring when the decal/stencil count is high. Just like pe you don't have to use all of it so it's what ever will meet your approval. 

The Academy 1/48 F4B has over 400 decals and stencils to apply. It took me several evening sessions to apply them. 

I thought the pin  striping on a black trans am was bad but a navy phantom beats all