Putting up fence using landscaping timbers?

Francis is right that you should try a post or two and then you will know! If you have not started yet I would suggest you not use timbers for your posts. Timbers are meant to be used above ground and will rot if used as posts. For not much more money you could get real treated fence posts that will last for many many years.

I would pour the concrete such that there is an equal amount as the thickness of the timbers on each side, or more.

For example, if the timbers are 6" think then I would pour concrete 6" all around the timbers, at minimum. 12" all around would be better. Timbers are very heavy so the need a lot of anchoring.

How many bags you need exactly is tough to answer. Do the bags tell you how much volume of concrete each contains? If you're going to dig a hole 3' deep, and 18" round, allowing for the volume of the timber you'll need about 5.33 cubic feet per hole.

If you're going to use 12" all around the timber, then you'll need about 8.75 cubic feet of cement per hole.

This is the sort of thing where it would be best to experiment with one or two holes first, and then figure out how many more bags of cement to buy.

I have used them and they are fine....3 years and holding. Just be sure to get the pressure treated ones so they last longer.