How common is the practice of polyandry here in America in the day and age of feminism?

Polyandry is marriage to multiple males, and typically it is fraternal in nature. I usually think of it in India, China, Tibet... Those areas.

You are referring to people having relationships with more than one person. That's been going on since the dawn of time, but the sexual revolution/birth control made it more common. Once women didn't have to worry about pregnancy we were able to have sex without worry. The obvious result is more men and women having sex with whomever they wanted to have sex with who was also interested.

Incels sure do whine a lot. No wonder they're unwanted.

not very common, probably less common than polygyny

It's not very common, hence its designation as a philia, and of course it's not state sanctioned on any level.