How many troops form a "large enough" force?

[ano]

I always wanted to know, how many troops you should attack with to make enemy mages use their scripts. If you attack with single scout, they won't cast scripted battlefield enchantments and other spells that require gems, but it may be really useful to send in a "large enough" suicide group of cheap chaff to see what is awaiting you.
Does anyone know what this "large enough" equals to?