In general, the person who creates a musical composition, the composer, designates how many, and which, instruments are to perform the composer’s work. In a typical work, an audience member may expect to see from 50 to as many as 100 musicians on stage. While the composer of a work may designate precisely how many people should be involved in the performance of their work, often the numbers are dictated by the era in which the work was written.
Baroque and Classical works were generally written for more intimate venues than Romantic, 20th, and 21st Century works, and were therefore more often played with fewer musicians. That is not to say that Bach and Mozart can’t be played with monster orchestras—they can—but more often, in an effort to achieve some semblance of the composer’s intent, they are played with small forces. By the mid-19th Century, composers were writing works that included many more players—more varied woodwind and brass instruments, lots of percussion, keyboard, harps. By the turn of the 20th Century, it is not unusual to need 100 plus players to cover all of the symphonic bases. (Contrast that with the 30-40 needed for a Mozart symphony!)
The age of the instruments in an orchestra can also vary significantly in age. The oldest instruments can be more than years old—and are always found in the string sections. String instruments, like some wine, and I am hoping people, get better with time. Unfortunately, woodwinds and brass don’t last nearly as long. In recent years, instruments made in the 17th and 18th centuries have been part of DSO concerts.