Good info above. I’ll add:
while Cornish x are more tender and similar to grocery type, I would have to disagree they are “mushy”. Grocery store birds often have adds, such as saline solution, etc, and I often find cheap grocery birds “mushy” but not home processed Cornish x.
Cornish cross have best feed/meat return, and have fewer feathers. More white meat, and some strains remain more active than others.
Dual purpose have their own positives that you cannot get from a Cornish, but I’d recommend sticking with known breeds good for quantity of meat on their frame, such as the list mentioned previously (New Hampshire, rocks, Buckeyes, etc). If you ever find yourself butchering a Leghorn, you’ll see right away why they are not considered dual purpose bc they are quite lean in the chicken world.
since you are new to butchering, I’d recommend 12 or less. We did 12 our first time and it took much longer than expected, and only 4-6 were hand plucked, the rest were skinned.
If you plan to butcher and throw away the bits and pieces, don’t do it the day AFTER the trash gets picked up....try to do it closer to trash pick-up day. Ask me how I know
Good luck.