Is it possible to search SO's based upon the number of fullfillments? I.e only pull orders with one fulfillment.

Yes. You would use a Summary Criteria of Count. Also you need to use SO Main Line = False as Fulfillments are at line level

can also go the other way. transaction type = item fulfillment created from : type = sales order main line = true results: group by - created from count internal ID as "number of item fulfillments"