The priority flat rate game board box will easily hold 2 keyboards, even big ones like Model Ms, and you might squeeze 3 smaller ones in, if they are thin enough.
The boxes are about 2 feel long and a foot wide, but only 3 inches thick, so while they are very roomy for even the largest keyboards singly, multiples will not fit end-to-end or side-to-side. All you can do is stack them, and even thin keyboards are an inch thick, seldom less.