I think about 5 to 6 members actively doing anything is enough to gain credits for a rank 1 company action. you can gain even more if your FC member decided to take on crafting, they take lesser time and you'll gain a few points for each item crafted. If you choose to use sparingly and activate then whenever really necessary (ie only use 5% exp when everyone decided to level the whole day), you will see that your FC credit won't drop below a certain level.

good luck with your FC by the way. =)