I thought that the 64bit version would give me access to as much RAM as I wanted,
Not 'exactly' "as much RAM as I wanted", but enough so that you don't need to worry about it.
the guy in the shop, a certfied Logic Pro specialist, tells me that it's not true, that the 64bit version has access to 8 GB RAM max,
Wrong. Logic in 64bit can access as much RAM as you can fit in any Mac right now... and more.
Your main issue is that it is impossible to upgrade the RAM on a Retina MBP after you have purchased it, so if you decide that 8G will be enough, rather than 16G, you will have to live with that for the life of the machine.
8G is good, 16G is better (or likely will be in the future.
Logic itself not using much memory and 8GB should be more than enough! But there's some software instruments like Native Instruments Kontakt that can fill up all your memory fast ) Im producing music and i can say that almost every project i made fill my 8GB to the top so i notice Page Outs in iStat menus. But since there is SSD in retina - page outs (writing to disk, when memory is full) not slowing process too much but still) My advice is 16GB anyway )