maxAtlasSize = GenGetMaxAtlasSize()
maxAtlases = GenGetMaxAtlases()
The maxAtlasSize ^ 2 is the maximum size of a tile atlas for the current device.
maxAtlases is the max number of atlases supported by the video hardware.
So max video ram is:
maxVram = GenGetMaxAtlasSize() ^ 2 * GenGetMaxAtlases()
Note that it may be impossible to utilize all of the vram since the heap memory available to Basic is far smaller than the vram on modern devices. You will get out of memory errors first.
The Game Engine allows up to 10 tile atlases, so you aren't limited to the max atlases. The Game Engine will swap in atlases to vram as needed. Since our atlas sizes can be any power of two it's easy to fit many atlases of various sizes into vram at the same time.