It's not that complicated. Strings are not like wind and brass where adding one or two players can make a big difference to the sound. First, strings can play really very quietly indeed and the more there are the easier it becomes to do this. Fifty players can be quieter than one soloist if they are all at a true pp. Second, good players have a very developed instinct for balancing with the rest of the orchestra. They know to play down underneath anybody with a more important part. So you can divide sections into almost any number of parts and the players will organise themselves to balance automatically as long as you don't do anything like pit one double bass against all the violins at forte. You can have one solo cello at the same level as all the violins though, or just the first desk of violins and all the celli. If you want one string section or sub-section to dominate a texture, just write them at a higher dynamic and keep the others out of the same register.
As so often, the best advice is to find scores containing what you want to study and listen with them. IMSLP is your friend.