The database sequence that handles the creation of customer numbers is defined in HZ_PARTY_NUMBER_S.
select max(party_number) from ar.hz_parties;
Then using the number returned above, define the sequence to start with a higher value.
If for example the above returns 10001, then you can do the following to ensure the sequence will not create overlapping Party numbers:
DROP SEQUENCE AR.HZ_PARTY_NUMBER_S;10002 INCREMENT BY 1 MAXVALUE 999999 CACHE 20;