No hard and fast rules really. Lots of different whys and wherefores.
Some general rules would be; earlier in the sengoku period probably means less of them, the richer the daimyo the more chance of a higher proportion of sashimono etc.
Also, for some larger battles, some smaller units could be integrated into a larger force and would have sashimono given to them.
No real heraldry rules either (not like medieval Europe). It was generally down to individual family member/officers in charge etc.
I’m assuming that the Samurai Archives are still up and running (haven’t been there for a while) and it’s well worth a look for guidance.
Basically, have fun and don’t worry too much about it
