Ah, I was looking at St Louis to La, not the return trip!
Baltimore was definitely considered as a new team - Cleveland Browns were just inactive for a couple of years.
In any case even with your numbers, that is eight changes in 20 years. In a (currently) 32 team competition. Seeing as we are not looking at expanding the league, that is like us adding/moving 4 teams in 20 years.
Lets think about what RL has done in that time. We added PSG, Catalans, Gateshead, Celtic Crusaders and their subsequent move to just being the Crusaders. So we are actually expanding as aggressively as the NFL. Even if you count the Browns as a move then new team. We just aren't as good at keeping the new teams alive!
You're comparing apples and oranges though. NFL is huge. RL is not. Expansion of the NFL is far easier to achieve.
Out of all those expansions, only one has worked. And out of all those expansions, only one was based in an area with a history of RL. What does this tell you? It tells you that unless you've got a butt-load of money (like at London), top-down expansion won't work.
So in 18 years of SL, we've got two expansion sides, and only one success.
So the suggestion of adding 3 or 4 clubs was in addition to the current 14 teams, or adding 3 or 4 clubs? If we do add one team at a time, where does the money to fund these clubs come from?
After looking back, the suggestion by the OP had 4 French teams and 6 UK teams so apologies there, although he has also suggested a SL2.
Either way, it's not something I agree with. The balance wouldn't be right. You can't have 33% of a league as new clubs. It's too dangerous.
Like I've said, one at a time would be better. Whether we replace teams or add to them would be dependent on the relative strengths of the clubs left. There are numerous C-License clubs at the moment that aren't pulling weight, so I wouldn't expand.
As for Wales, is there any potential single club there? And if so, do we add or replace?
As above, depends who are left. If you had 14 A-license clubs, you wouldn't replace them. Even 14 A/B-License clubs. But we've got 4-5 clubs on C-License (I'd say Cas, Salford, London, Wakefield and Widnes) so unless they improve, we should look to replace the worst ones.
Potential clubs are in Championship 1. They are just nowhere near ready. We don't need to rush this again.