Allotments 4 All

A basic question but here goes  :-[
I plan to plant a section this year for potatoes but find it hard to calculate the amount of seed potatoes i will need due to having very little experience.
The space i have allocated for them is 8m x 5m and thought of having two types "great Scot" and "Desiree".
Does this sound a reasonable size and type (more space can be calculated if need be) ??
If so how many should it approx' take. I remember last year seeing our local garden center having little paper bags of potatoes but unsure how many a bag contains?
Are there any websites that sell more of a bulk quantity out there ??
Thanks for any help or suggestions
-Martin-

With Scot a 2nd early and Desiree a maincrop they will take up different amounts of space. 3kg of seed potato is somewhere around 35-40 tubers.

A 4mt x 5 mt bed will take around 9 kg of earlies or 5 kg of maincrop which is about the right proportion ( 2:1 ) to plant.

Many allotment associations buy in bulk so it's worth trying some of the larger allotment sites in your area. We're buying in 800 kg which will be sold at £2 - £2.50 for 3 kg.

Thanks for asking the question. it was one I wanted to know the answer to myself.
Much appreciated info Tony  :)

tanks from me aswell i wondered the same being a novice