@titch_72There are many ways you can achieve that. Which method to choose depends on your list. Does it contain duplicate client names? Does it contain empty rows?
If your list of clients does NOT contain duplicates, you can use the COUNTA function. If it could contain duplicates and if you are a MS365 subscriber, your Excel might support the new UNIQUE function. Combine this one with COUNTA will count unique client names only. But note that empty cells within the selected range will be counted as 1 as well.
If UNIQUE is not supported in your Excel version, you can use COUNTIF in a helper column and then SUM that column. Alternatively, you could use a formula to assign a sequential number to each client, skipping empty rows, and then take the highest number (MAX) from the list.
The attached file demonstrates them all. I'm sure that there are more, and perhaps prettier, ways to do it. If non of these seem to work for you, please upload an example of your workbook showing the exact format and lay-out of your real schedule(s).