You can’t sacrifice more than one creature to any one exploit ability.

A creature with exploit “exploits a creature” when the controller of an exploit ability sacrifices a creature as that ability resolves.

If Colonel Autumn isn’t on the battlefield as an exploit ability of a creature you control resolves, Colonel Autumn’s last ability won’t trigger. You won’t get +1/+1 counters, even if you sacrifice a creature.

You choose whether to sacrifice a creature and which creature to sacrifice as an exploit ability resolves.

You can sacrifice the creature with exploit if it’s still on the battlefield. If it has an ability that triggers when it exploits a creature (or, in this case, when any creature you control exploits a creature), that ability will trigger.