When writing for strings, it’s important to be conscious of how many players are playing in each section for the group you are writing for, so you know that when using divisi how big a divided group will become which will change the volume of those groups. This again, will have its impact on the balance in the full string section.
I decided to go for a small orchestra sized string section which comes the closest to NotePerformer, the sound library that I use in Sibelius, getting this way realistic playback results:
Violins 1: 8 players
Violins 2: 8 players
Violas: 6 players
Cellos: 6 players
Basses: 4 players
Now, when you use divisi on a string group, you know exactly how many players will be playing on every voice. For example when dividing the 1st violins group into 2 groups, every voice on that staff will be played by 4 players.
In reality, you can encounter all kind of sizes, especially when writing for non-professional orchestras, so the above amount will be as good as any other sized group.