Looking for count of unique numbers, not total count of all number (if that makes sense)

I'm looking to identify the number of machines that had issues within any given month. The serial number appears in the raw data each time there is a single issue. So if SN 001 has 3 issues in March 2018, it's on 3 separate lines. If I do a Pivot table, under March 2018 (if SN 001 was the only machine with errors) it will show as a count of 3. Is there a way to change the settings of the pivot table so it will identify the count per discrete number....so in this example, it would return a value of 1 (because SN 001 was the only machine with an errors in March), rather than 3?

I've attached a file which may help to make more sense....this one is a little difficult to describe. Sorry!!!

Any help would be really appreciated!!!

Thanks!

Amy

Try this formula:

Formula:

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