According to your comment:
Well I'm trying to a add the phone contacts of a user. Usually they're ~500 per user.
The best approach in this case would be to create a document for each contact and add it to a collection. Your schema might look like this:
Firestore-root
|
--- users (collection)
|
--- uid (document)
|
--- contacts (subcollection)
|
--- contactId (document)
|
--- //Contact details
Remember, according to the official documentation regarding Cloud Firestore Data model:
Firestore is optimized for storing large collections of small documents.
Beside that, if want to store large amount of data in one document, note that the documents have limits. So there are some limits when it comes to how much data you can put into a document. According to the official documentation regarding usage and limits:
Maximum size for a document: 1 MiB (1,048,576 bytes)
As you can see, you are limited to 1 MiB total of data in a single document.