When ordering Ribs from a restaurant, they're commonly sold as a "full rack" or a "half rack."
However, that often doesn't tell the person how many ribs they'll actually get.
A typical slab or full rack of pork ribs contains 10-13 ribs.
Butchers sell pork ribs as "slabs." Unless you're at a barbecue joint buying ribs, you'll likely see them sold as a "full rack" or "half rack."
Depending on the breed of pig, they will have 15-16 rib bones. When a hog is butchered, 2-3 bones are left in the pork shoulder; Smaller bones are also trimmed with the discretion of the butcher.
In a grocery store, you should expect to see at least 10-13 bones in a slab or full rack of pork ribs.
In the above example, the baby back ribs have a total of 13 ribs. Meaning, if you were to order a "full rack" of ribs from a restaurant, you'd receive 13 ribs.
Granted, in my example above, rib bones 12 and 13 have little meat when sliced alone.
If I were to serve myself at home, I'd likely separate the bones between 11 and 12 and leave 12 and 13 attached.
When ordering from a restaurant (not a barbecue joint), the ribs you're likely eating are baby back ribs (loin back ribs). This is mainly due to the marketability of the meat.
However, if you are eating at a barbecue joint, they will likely make the distinction and state whether the rack is spare ribs or loin back ribs.
There are specific differences between spare ribs and baby back ribs, however, if a pig has 10-13 spare ribs, it will also have 10-13 baby back ribs.
In terms of serving sizes:
Loin back ribs come from the upper portion of the pig's rib cage - where the ribs and the spin meet.
Location of Baby back ribs on a pig:
Spare ribs come from the bottom of the ribs or the underbelly of the pig; Where the pork belly and bacon come from.
Location of Spare ribs on a pig:
Most folks who do barbecue would tell you that spare ribs are their favorite. Where-as people who tell you they like to eat "ribs" likely prefer baby back ribs.
Spare ribs can be sold as a whole "slab" or are often trimmed to what's called a "St. Louis Cut" or "Center cut." All this means is that the ribs are trimmed to remove the sternum, costal cartilage, and flap.
The above trim essentially turns the spare ribs into a more traditional looking "full rack" of ribs.
Spare ribs are meatier, and have more flavor than baby back ribs. However, they are definitely less tender than baby back ribs as loin back ribs are loin meat.
Pork loin is a very lean cut of meat as it's not used much by the pig.
Rather than simply taking my word for it, I looked at a few popular places that sell ribs and looked at their photos of "full racks."
Below is a quick breakdown of my findings:
*Just based on the photos on Chili's website, they essentially do what I mentioned above. They remove bones that offer nothing to the eating experience, ie. they don't offer much meat.
**This is entirely an estimate based on visual inspection as their photo is top-side rather than bone side.
Note: All of these restaurants sell baby back ribs.
For further comparison, I looked at a barbecue joint in my area called Big Fatty's BBQ. Big Fatty's sells spare ribs and doesn't sell baby backs.
On their menu they state the following:
*Again, a slab means rack - so a whole slab is a full rack.
Barbecue joints typically sell meat on a per pound basis and not based on the number of bones/ribs you received - hence why Big Fatty's says an average rib count.