Joined: Jul 2015
Posts: 7,476
Likes: 3,287
From: NW Oregon
Bikes: 1982 Trek 930R Custom, '91 Diamondback Ascent w/ XT, XTR updates, Fuji Team Pro CF road flyer, Specialized Sirrus Gravel Convert, '09 Comencal Meta 5.5 XC, '02 Marin MBX500, '84 Gitane Criterium bike
as FBinNY says, the balls must be the exact same size as what is in the headset now...
best to remove a bearing/cage, and take it in when you visit the shop.
as to bearing count if you go "loose"... put in as many as will properly fit, then remove one.
if there are too many, you will find no "sweet spot"... the headset will either be too loose, or will bind.
getting the correct ball/cage set will be easiest and best, IMO...