So the ability reads:
Battle Companion (major): At 10th level, you can summon a battle companion. This ability functions as summon monster IV with a duration of 1 minute, but for only a good outsider or an animal with the celestial creature simple template.
This ability can summon only one creature, regardless of the list used. For every 2 levels beyond 10th, the level of the summon monster spell increases by 1 (to a maximum of summon monster IX at 20th level).
I took it to read that even if a warpriest summons monsters of a lower level they cannont summon, say, 1d3 of the monsters. This seems obvious.
However, one could also interpret it as reading that you can only have a single monster on the battlefield at a time. Otherwise a warpriest, with nothing better to do, could spam the battlefield with numerous creatures (one per round, every round).
So, is it the first or both answers? Thanks!