Spareribs come from the belly and side of the pig, under where bacon (belly and side) comes from. They are about 2-4 pounds sometimes more, and are very meaty, but are less tender than loin back ribs. Spareribs tend to have more fat, more flavor, and cost less per pound than loin back ribs.
Pork ribs are sold in slabs. The number of bones in a whole slab will vary depending upon how the ribs were processed and trimmed. You should expect 11-14 bones in a whole slab of Ribs.
The size and weight of a slab is based on several factors, including the age and size of the Pig and how the meat was processed. You may hear terms like "3-1/2 and down", "3 to 5", and "5 and up". These are common ranges used to describe ribs for the weight ranges in which rib slabs are commonly sold. While these terms are commonly used by meat suppliers at the wholesale level, you won't hear them used in retail stores.
Most of us like to buy and cook whole slabs. Supermarket cut slabs in half for packaging convenience and to allow shoppers to buy smaller quantities. I find that it's easier to prep and cook a small number of large pieces versus a large number of small pieces. I also find that the end pieces can dry out, and you have fewer end pieces if you're cooking whole slabs.
People often use the terms "baby back ribs" and "back ribs" interchangeably. Loin back ribs come from the loin of the hog, where pork chops come from. You'll sometimes see them called baby back ribs or loin ribs in the grocery store. They are smaller in size 1-1/4 to 2-1/4 pounds and are less meaty, less fatty, and more tender than spareribs.
Because they are smaller, baby back back ribs cook faster than spareribs. They are one of the most expensive cuts of meat from the hog due to high consumer demand.
