Total variants in store determine your monthly subscription plan. Stock Sync is not like many other apps out there.
You are not limited to the number of updates you do monthly or daily. You have all the freedom.
Even if you have more than 1 update feed, the variant count will be the same if you are using both feeds to update the same products.
At first, you might be seeing ??? as the total variant(s) in your store. This is because Stock Sync doesn't have the count yet.