In flowering plants, each anther has . . . . . . . . number(s) of pollen sac(s).

In flowering plants, each anther has four numbers of pollen sacs.
The anther contains pollen sacs that produce pollen grains which are male gametophytes containing male gametes. There are four pollen sacs in the bilobed anther.
Two anther lobes contain 4 pollen sacs in which pollen grains are produced. Pollen grains develop from the diploid microspore mother cells in pollen sacs of anthers.