“Why does the string section have the most members in an orchestra?”
First, it depends on the work being performed and its interpretation by the conductor as to how many of all the instruments in the performance.
Second, consideration is the availability of musicians to perform. Some smaller cities or towns may simply not have 12 violins, 4 violas, 2 violoncellos, 4 cellos, and two double basses (or however many) not to mention the wind section, percussionists, and various soloists.
But you can ask the composers what will really deliver the impact of the symphonic work they composed – oh wait – they are mostly all dead – so, it’s back to the conductor whose deep musical knowledge will have him or her planning maybe two years ahead of the works they will present for programs in the future. If possible, many of the currently performing works could keep the current roster of musicians in jobs as the seasons turn through the year, or years to come, hopefully.
This may all be reliant on the popularity of the shows for ticket sales and fund-raising.
It is an expensive risk to throw a spectacle like an orchestral performance, and it never happens lightly.
All to often there are some near-desperate and existential financial goings-on behind the scenes. Wealthy donors are wonderful but may come with uncomfortable pressures with it. (Cheesy example) Mr. McMillionair’s mother-in-law always really loved “Stars and Stripes Forever” “so you guys would do well if you play it.” while the conductor and literally ALL the members of the orchestra would rather leave that for the high school band on the 4th of July (*ahem*) but must hold their collective nose and suffer. and while they endlessly pine for performing something interesting like Shostakovich’s Symphony #5 or some such but would struggle to sell tickets – and that’s tragic – people should get to hear all the profound deep stuff.
LOL – so I hope this gives an appreciation for why an orchestra has who they have.