Count the expected number of bids (after receiving them we close the
        contract. If the recipient type is broadcast return None which denotes
        unknown number of bids (contract will be closed after timeout).

def _count_expected_bids(self, recipients):
        '''
        Count the expected number of bids (after receiving them we close the
        contract. If the recipient type is broadcast return None which denotes
        unknown number of bids (contract will be closed after timeout).
        '''
        count = 0
        for recp in recipients:
            if recp.type == RecipientType.broadcast:
                return None
            count += 1
        return count