Hi Erin,
Registration questions are intended to be used to gather additional information about the registrant, such as dietary restrictions, if they need a shuttle from the airport, if they've attended the event before, etc. Therefore, it is not possible to put a capacity on a registration question.
You're correct in that you can achieve this by creating an optional session with a capacity. However, the event will not close for registration if this capacity is reached; the session will just appear as closed to the registrant, but they'll still be able to complete registration.
In order for your entire registration process to close once a capacity is reached, you would have to use the overall event capacity field in Event Details > General > Event Information > Basic Information. You can then choose to open a waitlist once this capacity is reached, if you'd like.
You could also create an optional item of "I am attending this event," with the maximum items per invitee set to 1. Under Status & Capacity, enter a Capacity of 900. Then, under Advanced Settings, set "Close the event if this item reaches capacity" to Yes. This means that if 900 people select this optional item, the event will then close for registration.
You could then add another optional item of "I am not attending this event," with no capacity set. However, these optional items will always display on the Item Selection page of registration; there is currently not a way to move them.
Best,
Julia