Its not related to flow I guess. But however I gave it a try and below is the approach

Formula in cell B1: =LEN(A1)-LEN(SUBSTITUTE(A1,";",""))+LEN(A1)-LEN(SUBSTITUTE(A1,",",""))+1
drag this formula till the rows you have data in column B.
This gives the count of emails in each row. Finally you can use =SUM(B:B) formula to get all the emails count.