In your situation I might keep the counter in another table of the same database. Make a Config table, and in one column of one row, keep your counter.
<pre>UPDATE Config
SET counter TO counter + 5
WHERE configItem='customers'
RETURNING counter</pre>
This does the update in one operation and returns a value which lets you figure out the ids for the 5 rows you want to insert. it is ACID, and threadsaf, given that you are correctly using separate connections.