I'm guessing the limit has to do with the fact that each mannequin can hold up to three additional items in the form of hats, robes and boots. Everything we store in our home takes up 'memory' on the servers, and we're sharing that space with millions of other wizards.
There's also a limit on the number of snow globes we can have, and we're limited to only one seed vault per home.
Alia Misthaven