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!
I always took it to mean only one creature per summoning but you could use it multiple times.
There are abilities that forbid having multiple summons out. The are worded differently than this, and they pre-date this. If the ability was meant to disallow stacked summons, it would say so.