Now the question is whether having more LWers makes it easier or harder to recruit new ones.

If the model is "only certain % of population is the LW type", then it should be harder, because the low-hanging fruit is already picked. If the model is "rationality is a learned skill", then it should be easier, because the existing group can provide better support.

I already think Finland is a very smart country (school system, healthy lifestyle), so if it's the latter model, your local rationalist group should have a great chance to expand. It's probably important how many of the 15 Finnish LWers live near each other.