Well, actually, not that much. Let's make the calculus.
Solaris has, IIRC, 2 textures. Each of them has a diffuse, a glow, a shine and a normal map. So we have 8 maps on the Solaris.
If we have an animated background with 30 frames, we're at 30*6 = 180 maps.
So we're actually at 22.5 Solarises, minus polies. 5.6 Solarises if they're using 4096² maps versus 2048² for the skyboxes.
I'm not convinced this comparison is very pertinent. Ships in general will be much more performance-eating than skyboxes, with polies, collision detection, weapons...
EDIT: Also, keep in mind that even if you put 22 Solaris on field, maps will be loaded only once on the memory. You'd need 22 Solarises equivalent with different maps each. Have fun with that.