If you want multi-GPU support, it's best you buy an X470 board. B450 does technically support AMD CrossFire(2 AMD GPU's), but the support is not proper and you might face bottlenecking(more so than is normal with Crossfire and SLI). It does support your 1080, but don't try 2 1080's, that's not supported by B450, you definitely need X470 for that.
In short: On the B450 board, single GTX 1080 is good, double GTX 1080 is not good, single AMD is good, double AMD is supported but should be avoided.
On X470 board, everything works. But since this is more expensive, buy this only if you WANT to do multi-GPU, otherwise buy B450.
P.S: Avoid using multiple GPU's of any brand, always buy the best single card you can buy and be happy with it...