There are tens of thousands of Mormons in small sects like FLDS. They are based in the Intermountain West and British Columbia.
There are uncountable numbers of Christian groups and families and non religion based families as well. These people are widely spread.
More and more immigrant men are importing extra wives from their home country. Estimates begin in the tens of thousands. They are mostly in large cities.
Since polygamy is illegal in the US, these people live under the radar and don’t stand up to be counted.