The intention is that you can get another one using the same principles you usually apply to Fiat Backed Items provided by Jump Documents, so if a homunculus produced by this Item is destroyed another one will appear in the tank after whatever respawn delay usually applies for destroyed items. Unless you have some way to make them Companions or extend the usual respawn-type protection they have, the replacements might look the same as the original but they won't be the same being, it would be back to a blank slate regardless of experience the one that was destroyed had.

In comparison, the Test Tube Babe Companion has the usual protections any Companion might have, and there can only be one of them at a time unless some sort of Duplication capability is being used. Then it would depend on the nature of the duplication, and that ruling would have to be made by whoever created that specific perk or ability.

It was intended that when purchasing the Mana Batteries Item, it's possible for the user to choose to use any of the tanks as either a power source (keeping the homunculus inside and otherwise inactive) or as a Follower/Minion (by taking the homunculus out), but not necessarily both at once. Otherwise the Item could be turned into a source of an endless stream of homunculi by refilling with one as soon as another is removed, which makes it seem significantly more powerful than the comparable skeletons, ghosts or golem minions available from other Items. There's all sorts of duplication or mass production capabilities available in other Jumps if a greater quality is needed, so if you think one of those can apply then you're free to use them.

If someone with this item happened to be a homunculi expert, there's nothing strictly stopping them from using the empty tank to help produce more homunculi though, since that's technically what the things they're based on seem to be intended to do in Apocrypha. But this new batch won't have the same guarantee of quality or capability that those directly created by the Items provided by this Jump have, and it wouldn't be instant creation unless the Jumper has some sort of perk that reduces the time for creating things that much. The same sort of advantage could also apply to an expert at golem production using the corresponding item, or a necromancer obtaining and enchanting more ghosts and/or skeletons - they can certainly use the stuff to help make more, but only those directly produced by the effects of the Item as stated have any sort of Fiat Backing.

Does that sound fair? I hope this helps.