Yes, if you exploit a cheesy RAW loophole
By RAW, yes - I can see an obvious loophole which would allow the battle smith to actually accumulate at least as many steel defenders as they have spell slots at once, depending on exactly how you weasel the wording. The critical part is that:
If it has died within the last hour, you can use your smith’s tools as an action to revive it, provided you are within 5 feet of it and you expend a spell slot of 1st level or higher. The steel defender returns to life after 1 minute with all its hit points restored.
At the end of a long rest, you can create a new steel defender if you have your smith’s tools with you. If you already have a steel defender from this feature, the first one immediately perishes.
You can only create a steel defender at the end of a long rest, and if you choose to do so the first one you already have immediately perishes - a.k.a it dies. But nothing in the rules prohibits you from immediately using your action to revive the steel defender which just died (within the last hour), and that does not stipulate that any other steel defender you have dies. Now you have two steel defenders. There is no rule that says you can't have two at once, or that you can only have one at a time - only the rule that specifies the first one dies when you create a new one.
You can then take a long rest and create a new steel defender. Depending on how you read the ability, now either only your first steel defender dies, in which case you currently have two, you can revive the first and now have three, and you can continue making new steel defenders with long rests in this manner to accumulate as many as you like - or, if you interpret it to mean that all your pre-existing steel defenders die, you can revive as many of them as you have spell slots, which would ultimately allow you to have up to your total spell slots plus one defenders.
Of course, this is very obviously not how this feature is meant to work. The clear intention is that you are only meant to be able to have one defender at any one time, and no reasonable DM should let you abuse the imperfect wording used in the feature to accumulate more than one steel defender (unless, of course, your group enjoys this kind of system-breaking and are happy playing that game!) Since the game does not expect that you should have multiple steel defenders in play at once, the rules don't specifically clarify how many individual defenders you could command with a single bonus action, but by a strict reading I'd say you could only command one at a time.
Aside, you do appear to have misread the Artillerist's Eldritch Cannon. I cannot see anyway you could justify having more than one cannon off the bat, since in that feature's wording you explicitly can only have one at a time and can't make a new one if your previous one still exists:
You can have only one cannon at a time and can’t create one while your cannon is present.
And they disappear after an hour anyway, so you can't stack them up with long rests even if you could make more than one. Only the Fortified Position feature allows you to have more than one cannon at a time, and even then that still explicitly limits you to two.
