You're probably going to get "call a pro" but I'll hopefully give you enough info to ask a question someone can answer.
Conductor size depends on how much current will be going through the wires which depends on how much current the device at the end of the wire needs to operate. The more current, the bigger the wire needs to be. Telephones only need very tiny wires. Your Electric dryer needs fairly large ones.
There are lots of conduit sizes too and how many wires can fit inside any size depends mostly on what size wire you're using.