"I've read anything over 50# is 14 strands, under 50# is 12 strands."
Depends on the string material and your personal preferances. That's a good rule of thumb for dacron. Over 65-70#, I'd go with 15-16 strands.
"I understand a recurve get 4" longer than the bow and a longbow gets 3" longer. Does the same go for a self bow? Does it matter?"
That's the old rule of thumb. AMO says 3" shorter, longbow or recurve--actually it says the bow should be marked as 3" longer than the "string master" (steel cable) that puts it at proper brace. Neither are 100% reliable. I take orders by actual string length, as there are lots of bows that don't follow either "rule". I've made recurve strings anywhere from 2.5" to 6" shorter, longbow strings from 1.5" to 4" shorter. For the selfbow, 3" shorter is a starting point, but may or may not be on the money.
"...can artificial sinew be used for string making?"
It can, but it makes an awful string. Stretches like crazy.
Here is a link to the AMO specs.
http://www.texasarchery.org/Documents/AMO/AMOStandards.pdf I've made well over 10,000 strings and been involved in two string-making videos...and I'm still learning things.
Chad