You do your washing everyday and will never go to sleep with dirty washing in your dhobi bucket, so enough pairs to wash, dry and wear really.
Bear in mind you will be issued socks for day to day wear (CS95s, drill, PT etc) and will only wear civvie socks on runs ashore and possibly in the evenings while doing admin.
As for boxers you may find you start to go 'commando' as boxers end up being something extra to wash, an extra layer of wet clothing in the field, something extra to change 3 times a day etc...
Basically, use common sense. Dont bring shed loads with you and dont bring just the one pair thinking you can turn them inside out halfway through the week.