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To my understanding, an Artillerist artificer can create an Eldritch Cannon during a battle using an action. The first one of the day doesn't require expending a spell slot.
But what about activation and movement during a battle? Can you activate and move all Eldritch Cannons using the same bonus action?
The description of the Artillerist artificer's Eldritch Cannon feature (Tasha's Cauldron of Everything, p. 17; emphasis mine):
Once you create a cannon, you can't do so again until you finish a long rest or until you expend a spell slot to create one. You can have only one cannon at a time and can't create one while your cannon is present.
Spending a spell slot to create a cannon is how you replace it during the day if the initial free one gets destroyed. You can't just keep spending spell slots to produce more & more cannons to create an army of them.
The description of the 15th-level Fortified Position feature says (Tasha's Cauldron of Everything, p. 18; emphasis mine):
You can now have two cannons at the same time. You can create two with the same action (but not the same spell slot), and you can activate both of them with the same bonus action. You determine whether the cannons arc identical to each other or different. You can't create a third cannon while you have two.
Once you hit level 15 and gain the Fortified Position feature, you can have two cannons out, and a single bonus action can be used to command both of them.
