You can access through material- and entity skins a total of 8 textures. Using one texture as mask this means that you have access to a total of 7 texture á 4 channels => 28 monochrome texture maps which can be divided into 5 pairs of RGB textures and 5 XY normal maps (for texture, the z-component can be restored easily through arithmetics). This means, assuming you use it on terrains and / or entities, you can have a multitexture normalmapping shader with 5 texture + normalmaps max!

Since you have access to static bitmaps and if you are only using ONE terrain at a time with this shader, you can, though, use unlimited texture/normalmaps, of course, if you use then static bitmap references.

Nevertheless, there more texture/normalmaps you use, the longer calculations are to be taken into account. Are you sure that you need 5-8 textures? Can't you break your environment into pieces and use textures only where they belong?