How many documents can a single collection have in MongoDB?

I haven't found any information about this.

How many documents can a single collection have in MongoDB before it is necessary to use sharding?

Can a MongoDB have multiple collections?

As mentioned above, a single database can have multiple collections.

Can MongoDB handle millions of data?

Working with MongoDB and ElasticSearch is an accurate decision to process millions of records in real-time. These structures and concepts could be applied to larger datasets and will work extremely well too.

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.