Stafford Lannister’s forces at Oxcross consisted of 4,000 veterans of the Battle of the Camps who had managed to get to the Golden Tooth (source) plus a number of “sellswords, freeriders, and green boys from the stews of Lannisport,” and since the wiki had given 10,000 as the overall size of the force at the time, 6,000 were those latter soldiers.
While the wiki has since changed, 10,000 still seems like a sensible number given that the latter part of Stafford’s army was referred to as a “host” and GRRM doesn’t use that term to describe small forces. (Stannis’ 5,000 men are described as a “meager host” for example.)