How many do you carry? And do you drop one every time there’s a backward pass beyond the LOS, or only on this type of play?
When I worked deep (S, F, B), I would have two bean bags on my belt. Had to use both on a few occasions. Not for multiple fumbles, or multiple backward pass, or combinations. But, those DO happen, and yes, you need to mark each of those spots with a “Spot Marker,” informally called by what it is - a bean bag. I had to use two occasionally when I would have an illegal touch, then the end of the kick, during a punt play. Those happen with some greater level of regularity. If you run out of bean bags, then use your hat.
Now, in those highly unusual instances of multiple backward passes (or handing) - more than 3 -, you may run out of spot markers. We can only do what we can do. Often, one, or more, of those ‘backward’ passes, is actually forward, so that spot gets marked with a foul marker (flag), or spare bean bag, hat, whatever it takes. It is possible you’ll just have to make a mental note of the yard line at which a pass (forward or backward) was thrown.