It isn't clear if summoned creatures add experience at all
So, you are jumping to "How much does it add", but we don't even have the answer to "Does it add?". Check How does the summoning ability affect the encounter difficulty? - note that we actually don't know if the summoned creatures are expected to be considered in the creature's CR already, if it should be added or what.
From my point of view, a creature that can cast Animate Objects shouldn't, at least not without playtesting, have a higher CR than other spellcasting creatures that can cast other 5th level spell, with the same stats. But there is also the fact that increasing the number of enemies changes the action economy fundamentally, since now there are more enemies.
Possible Mearls talking:
In ENWorld, I've found this thread that mentions an answer by Mike Mearls, stating:
Q: For monsters that can summon in help, like the pit fiend, do the summoned monsters count toward the pit fiend’s XP, or must they be accounted for separately when building encounters?
A: If a monster can summon other creatures, that ability will be accounted for in the monster’s XP value, so the DM won’t need to make any adjustments.
Sadly the link is broken, so I can't verify. Also, Mike Mearls answers are not official, even though he is a developer.
Current DM's Basic Rules
From the DM basic rules, in the Experience subsection:
Unless something tells you otherwise, a monster summoned by a spell or other magical ability is worth the XP noted in its stat block.
Animate Objects doesn't state a CR, though.
For now, it's not clear that we should add any experience at all. Personally, I don't see why a creature casting Animate Objects should give more total experience than one casting Cone of Cold or Cloudkill. So, essentially, zero.