We used two 4' sections of 12" deep shelf because that was the only way we could buy 12" deep, we really just needed 6'. So, we have some scrap, and it will get hung down in DH's workshop in the basement as an extra shelf. No biggie.
In hindsight - the kitchen upper cabinets are actually 16" deep, so you could use 16" deep shelving on that side, but the dinette cabinets are only 12" deep, so you'd have to buy 2 different depths if you wanted to do both.
What stores do you have near you? Target/Walmart sometimes carry this type of shelving.