How many cells are in the interphase stage?

About 90-95% of all cells.

For any given number of cells, about 90-95% of them are in interphase. This is because among the different stages in the cell cycle, the interphase is the ‘˜resting’™ stage or the phase where the cell is not undergoing any replication or cell division, which is about 90-95% of the time for most cells. This is according to a handout from the Division of Biological Sciences of the University of Montana.