That means they are seedlings and as varied as brothers and sisters in all species. It's what you would have ordered if you had thought about it in time. You're building a forest, not a hedge. Ehhh?
Since they have already been planted enjoy the season and don't make the decision now. If and when you do separate them, wait until spring next years.
Since they are Japanese maples which are highly variable, it would have been best to plant them in different pots for a year or two as suggested above.
Its all good, just a process that starts with a single step.