Don't know how I come to this topic. I am mostly interested in music composed between 1770-1800, and by the generation of composers born from 1730-1760 (roughly between Haydn and Mozart). Data from appendix of 《Cambridge Guide to Orchestra》has data on orchestral compositions in history - the ones listed with 1770's date (Milan Regio Ducal Teatro, Paris Concert spiritual, and Mannheim Court Orchestra all have 45~60 members. Between 1781-1805 the number was between 23 for Eszterhaza (where Haydn was) but mostly 30-40 (Leipzig Gewandhaus-Orchestra; Vienna Burgtheater, London Salomon Concerts in 1791, and Dresden Hof-Orch). There are also two records of huge orchestras (40-50 violins each) for unspecified orchestras in London.
For small classical (really, meaning those plays music for the last third of 18th century) chamber orchestra, I think the way to construct one is think about this as:
16 strings - 10 violins, 2 each for violas, cellos and double basses (2 of the violins could be five-strings, that does give some flexibility in sound balancing)
8 woodwinds - 2 each for flutes, oboes, clarinets, bassoons
4 brass - 2 horns, 2 trumpets
2 others - 1 timpani/percussion, 1 keyboard