For a quick way to count rows that have data, select all the cells in the first column of that data (it may not be column A). Or click the column header to select the entire column. The status bar, in the lower-left corner of your Excel Online window, will show you the row count.
Do the same to count columns, but this time click the row header at the left end of the row.
Here's an example of what you’ll see in the status bar:
If you select an entire row or column, only cells that have data are counted. If you select a block of cells, the status bar count shows the number of cells you selected. If the row or column you select has only one cell with data, the status bar stays blank.