Yes that's a "per unit" limit.
But the number of units is not stated, up to the discretion of the officer.
I doubt if a half-dozen would be refused.
I doubt two dozen would be allowed.
Putting them all in a special container may well reduce the number, IMO better to have a few here a few there but not making it obvious you're trying to be deceptive.
Check out Grin's modular LiGo battery