Hi GW
I have been to only one before, I ended up buying about 18 different varieties. Yes, you may buy only one of a variety if you so desire. My advice would be to have a clear idea of how many spuds you will have space to plant, divide that number into how many of earlies, second earlies and mains etc, you want to grow. Make sure you write on the bags the name of the spud and whether they are first early etc- whilst in the shop- or you will never remember!
I also enjoyed trying varieties I had never heard of before although, this year, if I can only find a seed potato day in Berkshire, I mean to reduce the number and plant more of the ones I know work well on my plot.
Hope that helps.
Z