there is no way to tell how many dbs unless your going to use the market, wich can turn out to be expensive. i suggest twin, for no other reason then its my preference and that there is a possibility you will save money.
things to remember tho, always use lucky time when uppin ur gear, not just for a chance for a 2nd soc, but also for a chance for it to increase in quality while you repair from a failure and for the oppertunity to avoid dura deterioration. lastely also realize that when you rb your gear, ur dura is higher than it should be for a normal un- rbed piece of equivalent equipment. this will lessen the chance of your dragonballs effectiveness. your best bet is to upgrade it 1 lvl with a met first to get the dura back to what it should be, and then throw a few dbs at it. dont expect them to work everytime, i have gone as many as 11 consecutive times on the same item in twin without upgradeing quality. its just a game, dont kill urself over a few bucks.