All the teams in the SPFL are professional. That's what the "P" stand for.
If you mean we don't have enough full-time professional teams, we have certainly had part-timers in the top flight before.
I wasn't being completely serious with 18 teams (although throw Hearts, Hibs, Rangers, Falkirk, QoS & Raith Rovers into the mix, and we wouldn't be far away). However, I'd rather see a league where we played each other twice and had a winter shutdown than a league with a split and a 7 game January fixture list.