As the spell is not strictly a touch spell, one can argue that you're not holding the charge, and so regular rules would apply - you can have several "sacs" at once. This is basically reasonable for 3rd level spell...
However - some things are undefined.
While poison typically comes in 1 dose vials, what if you had multiple doses in one? Does this mean that every dose available (within some reasonable limit) was in 1 sac?
While you may choose to deliver it with a touch attack, can you release EVERY sac at once in a single attack?
I believe the rules are vague enough that this is largely house ruling and opinion, and I've already got the official GM's ruling, so no need to suggest asking the GM. This is a thought experiment for those who haven't yet done so.
Since when delivering the potion, the potion is the thing doing the damage, not the injection (and thus the attack), should sneak dice be applied when applicable?
Touch Injection makes no reference to "vials" only to "poison" and ingested poison (but not contact) could be stacked...so...yes...if you hold more poison, you'd inject more poison.
I think you can only deliver poison from a single sac, since you are releasing what was created by a single spell.
Sneak attack dice aren't applied as it is a melee touch attack which usually doesn't cause damage by itself. Striking as unharmed attack would work, but would change it and no longer be a touch attack...
Skarm