Perhaps this will help get a feel of Re
If memory serves me right (and it is known to be suspect) a full scale F-15 at some flight conditions operates at a Reyond's number of 30-40-50,000,000.
Reduced to a 5% model for wind tunnel testing we have found that using a wind tunnel capable of 1,000,000 Rn will give barely acceptable results in drag. We would chose to avoid using this type of tunnel if we could.
In a wind tunnel operating at 6,000,000 the drag results are nicely applicable to full scale.
The wind tunnel model is very carefully duplicated out of steel and aluminum. Exact airfoil copied, etc. They did great work. It is necessary though to add "transition strips" on the wing and tail surfaces. This is some form of fine grit or similar small objects laid in a strip along roughly the 5% chord point of all surfaces.
We have models in RC that are working at such a low Rn that it no longer duplicates flight conditions. In a greater sense it becomes something that unless you are working in the exotic FAI glider class or models it is pretty useless. I am impressed by the work of Eppler and the others in the low Rn airfoil work though. The other writers in this forum have covered the other aspects of the number pretty nicely.