I need to know how many cells I have selected in Numbers

Use the bar of formulas at the bottom of the screen.

If all of the selected cells have something in them (none are blank), COUNTA will give you the count. If COUNTA is not there, use the gear to include it. It will give you a count of all non-empty cells in your selection (of 2 or more cells), but it will not count any empty/blank cells.

If all of the selected cells are blank, with absolutely nothing in them, you can do the same as above using COUNTBLANK.

If your selection is a mix of filled and blank cells, you can get a full count by looking at both of those functions. Just add them together in your head. It might be a good idea to do this in any case because sometimes a cell that looks blank may not actually be blank. For example, a space character looks blank but it not.

I now see from your signature that you are talking about Numbers for iOS. This is the forum for Numbers on MacOS and my answer was for the MacOS version. I do not know if the iOS version has anything similar but I doubt it.

COUNTA is there. Include COUNTBLANK by clicking on the gear.