Good morning,
I apologize if I'm not understanding correctly, but if you want to find a count of all the items that contain "SN" you can use this:
=COUNTIF(A:A,"*SN*")
Or, of you want all the SN's that are not paired with PT or OT you can use:
COUNTIFS(A:A,"*SN*",A:A,"<>*PT*",A:A,"<>*OT*")
Hope this helps. Best of luck,