There must be times though, when you need to indicate on a short score how many violins will play a particular passage. If we talk 'BH' then the Prelude probably uses the whole component, obviously there are solos here and there, like the added violin on 'Farewell to Rome', but there may be passages where just, say the effect of just six of the violins are what a composer wants. How easy it then to suggest this on a two-stave score? Is it a matter of saying '2nd. vls 6' or whatever?

This is very interesting, because we assume a full symphony ensemble will give a richer sound. But perhaps a composer wanting to scale up for a concert or album might actually prefer to rewrite a whole piece if the effect will be so different.