I believe the answer is 36. This is disturbing, because if the answer is not 42, you are asking the wrong question.
221 = 31 x 7 + 4 : make 31 new bottles, with 4 left over
31 + 4 = 5 x 7 : use those 31 and the leftover 4 to make 5 new bottles
you are now down to 5 bottles, so you can't make any more
this is assuming they get the bottles they make back after they are used