[Cementjungle]

Therefore, 12 is the first acceptable answer.

What happens if she has 13 friends?

[stormhill]

Any answer is wrong because there simply isn’t enough information to solve this problem. I’m inclined to believe this is a hoax.

[dr_lew]

It is stated that she has determined that she will need either 4 or 6 bags. This is then a “given”. A secret protocol, perhaps.