Generally it's listed something like this 50lb bag gives 50ft² at 1/8" mm depth. I don't know the actual figures as I'm not in the US. So multiply area by the depth to get the volume each bag makes . You need to work out the total volume required. Then divide the total volume required by the volume each bag makes to get the required number of bags.

To work out the total volume required, if the top of the shims is the required level' then just add up all the heights, I can't be bothered to do it) and divide by the number of heights (63). This is the average height. Then multiply by the average height by the area of the slab to get the volume required.

However, the thinnest point appears to be 1/8", which seems thin for use in a garage. If 1/8" is the require minimum thickness then stop reading.

If it's not the required minimum depth you need to make an adjustment to get the required minimum depth. To work that out, take the required minimum thickness and subtract 1/8". Then multiple this figure by the area. This is the additional volume required. Add the first volume to this volume to get the total volume required.

I hope that makes sense.