"It doesn't matter what size the bag is" Right.
A 50 lb bag holds more sand than a 40 lb bag If you want to know how many bags, you need to know how much sand is in each bag.
If you don't understand this, then I can't help you.
Go buy 4 begs. I assume a beg holds the same amount as a bag so start there. If it's not enough, go buy more. If it's too much, sprinkle the left over around the garden. It'll help with the drainage.
Also, the Home Depot by me sells 40, 50 and bulk sand. So the size does matter.
Just take the empty base with you and tell the guy at the counter "Fill er up!"
No no how many bags of wet sand (50lb) will it take to fill my bob up to the point he wont move around? I wanna buy wet sand but before then I wanna know how many 50lb bags of wet sand should I buy in order to fill central bob up?