Can an excisional lymph node biopsy of begnin be accurate when the node didn't have enough cells to do a flow cytometry?

Yes: If the the lymph node biopsy is adequate for evaluation in terms of sample size for the pathologist, then a benign lymph node diagnosis should be adequate even in the absence of flow cytometry data. Flow cytometry is helpful as ancillary data when working up a diagnosis of lymphoma or leukemia in conjunction with the biopsy. But if the biopsy is benign it doesn't add much.