Correct.
There's only meant to be one Prime at a time, as far as I know, thus there is only one spark in the canister at a time, as a dying Prime (IE Optimus in TF:TM) hands the canister on as their spark rejoins the Allspark for good, and the new power up comes when a new Spark is 'chosen' by Primus (IE Hot Rod in TF:TM)