if you want it to last, 5 to 6 inch with rebar or steel wire mesh. make sure it is in the middle not sink to the bottom when you pour the concrete, you can use a hook to pull them up if they do sink, but it is better to build a support first.
Really depends on how much loading you want it to bear. Usually, 2-inch is enough. Again, strongly recommend pavers, which drain water better. In case one cracked up, you only need to replace that one.