How many documents can be store in a single bucket?

I am using single bucket for a large amount of data(more than 2,000 Million rows) so it can be problem or its cool with single bucket. or please suggest best way to implement it which help me for fast searching , update, and insert records with these type of data.

You are ok storing large number of keys in any bucket. 2B would be fine. We do recommend that you follow the sizing guidelines to get the appropriate HW for your processing needs.

What to do when you have 10-20b or more ? What to do then ?

see Size Limits | Couchbase Docs

There does not appear to be a limit on the number of documents in a bucket.