Below are the sand bag count recommendations based on tarp size. The spacings are one bag every five feet on the perimeter and ten bags down the middle:

24x100=60 bags

32x100=64 bags

40x100=66 bags

50x100=70 bags

If your area is not windy, you may be able to get away with less.

Sandbag weight averages 10-15 pounds depending on amount and type of fill material.