SQL to increment 'identity' column for multiple rows in one insert statement?

I have a column in a particular file that I need to keep unique. I also
need to add a batch of records to that file. In testing, I currently
read into a host variable the current max(index)+1 to get the next
available one and then use that in my subsequent insert statement.
Naturally, when I do I get the same index # for each record. How can I
do one insert statement and guarantee a different index for each?

I thought of doing a (select max(index)+1) as part of the insert
statement itself, but was concerned that it would only evaluate that
once and not for each record. There is no commitment control in use.

Here's my code:
...
exec sql
select max(vfindex) + 1,
into :index
from mylib/mylf;
...
exec sql
insert into mylib/mypf(vfindex,
vfpnbr,
vfmodel,
vfgroupset)
select :index,
subpart,
modl,
:grouppart
from mylib/mylf join mylf2 on subpart = part
where part = :grouppart and atyp='F_LVL';
...

Thanks!