Overdubbing Small Line-Ups
Overdubbing the same instruments to create a bigger and more epic sound only works to a certain extent. While it is possible to overdub a quite large string section to give it a little bit more substance and power against the brass/percussion, trying to create an ensemble sound from a solo string instrument by layering it several times will not work convincingly as the vibrations and resonance in an acoustic space behave very differently between the individual instruments than with digital addition. Also, you would need to change the seating on every pass in order to create more sonic variety.
The bottom line is, that overdubs will never sound as big as if you had the same amount of instruments in real life. Still of course, in situations where there is not enough budget it might be a wise decision to try to get the sound a little bigger with overdubs (and/or adding samples). However, in some countries music unions forbid doing such a thing and you always need to calculate whether costs of the overdub time are cheaper than actually hiring more musicians.