How do I use validation rules to only allow one input in multiple cells?

Originally Posted by ChemistB

Uncheck "Ignore Blanks", select your data range (I assumed C2:J12) and go to Data Validation>Custom and enter this formula

=COUNTIF($C4:$J4,"Yes")<2

Only 1 "Yes" can be entered per row. Is that what you are looking for?

Thank you thats exactly what im looking for!