Spuds in sacks/planters, how many?

 JBA's getting started in bags below from their website with video ...
http://jbaseedpotatoes.co.uk/info/help-and-advice/growing-potatoes-in-bags/

Following up from Totty, my bags aren't put in trenches (as I bag to save ground space) and are sited on a soil path. I feel an experiment coming on ! 

Totty, do you grow like this for show only, or also for the kitchen?

Just one per bag. What size bags totty ?

tangojulie's post has pretty much confirmed what I was thinking as some people plant 1, some 3, some 5 with no noticeable difference in yield. You can only fit/grow so many spuds in a sack regardless of the number of seed potatoes used. Once it's full, it's full.

I am still going to try a few sacks with 5 in each, both 1 and 2 layers to see if they're any better, but my main method will be 3 max per bag. My thinking being why put 5 in if I can get the same amount from 2 or 3? I can therefore start more sacks for the same amount of seed potatoes, resulting in more spuds overall. This is why I asked about the 2 layer method as you would think that would be the same as having 2 sacks!?!

I don't know, I keep changing my mind! I'm good at indecisiveness sometimes.  .....or am I, ha ha.

if you do a lot in bags wont it cost a small fortune in compost.