Keeping Count only targets a single other copy of Keeping Count when calculating the attack bonus. So, if there were 3 copies of Keeping Count in play, one on Glorfindel with 5 tokens, one on Elrohir with 3 tokens, and one on Elladan with 1 token, then Elladan could get at most a total of +4 attack strength from Keeping Count (4 being the difference between his 1 token and Glorfindel’s 5).
Cheers,Caleb