How many documents can a single collection have in MongoDB?

@M-A.Fernandes has right.

I can only add this information:

Maximum Number of Documents Per Chunk to Migrate

MongoDB cannot move a chunk if the number of documents in the chunk exceeds either 250000 documents or 1.3 times the result of dividing the configured chunk size by the average document size. db.collection.stats() includes the avgObjSize field, which represents the average document size in the collection.

There is no limitation as you can see here:

If you specify a maximum number of documents for a capped collection using the max parameter to create, the limit must be less than 2^32 documents. If you do not specify a maximum number of documents when creating a capped collection, there is no limit on the number of documents.