Motherboard has four RAM slots - - total supported RAM is 32GB.
32GB divided by 4 = 8GB
So each slot supports up to 8GB - - simple arithmetic my friend. Info is right there on the manufacturer's specs sheet:
https://www.asus.com/us/Motherboards/CROSSHAIR_V_FORMULAZ/specifications/
However, getting another matched pair of 4GB sticks (or a matched pair of 8GB ones) would be preferable to enable all the installed RAM to operate in dual channel mode, thereby maximising RAM performance.