Its not an easy question to answer and at the end of the day it is experience that will determine what blade you use for the job in hand. As a guide though and based on my own experience I would use a No 3 blade for fine detail work such as internal cuts on fretwork in plywood. I use 18mm hardwood 99% of the time and I often use a No3 blade for some of the names I do on plaques where the words can be quite small. For most of my internal work I use a No5 blade and for cutting the outside of patterns I wwill use a No7 blade. I have many other blades but very rarely use them and the only blades I use now are Flying Dutman ultra reverse having tried all the rest. Going back on what I said a lot depends on the density of the wood, kiln dried hardwood is harder than air dried and species like Oak and Ash can be challenging. The only pine I use is southern yellow pine and I will use any of the above blades depending on the pattern and how small the internal cuts are. If you would like to see the sort of work I do you can look up my facebook page, wooden crafts from Wales.Hope this helps.