The average human anus can stretch to about 12cm, considering the pelvic girdle and the muscle in between. Doing some calculations, you get an area of around 452cm2. I don’t know how large the width of an average gummy worm is, but I’d assume <1cm, and factoring in its squishiness, I’d roughly double that number for a grand total of \~900 gummy worms you can shove up your ass.