what is the maximum number of concurrent access to Cauchbase?
what is the maximum number of concurrent access to NoSQL(e.g Cauchbase) databases?
Give example of NoSQL DB in practical use today with reference
who provide hosting service for NoSQL apps?
I do not know how to answer this question, because it is almost impossible to give a unique answer.
The only real limit to this is your hardware/topology. You will find several projects where the Couchbase cluster responds to multiple hundred of thousands of operation per second.
You can for example look to the Couchbase Benchmark based on the YCSB that give you some numbers:
http://www.couchbase.com/presentations/benchmarking-couchbase and the Couchbase sizing chapter.
Remember that Couchbase database and client(s) have been build on a 100% non blocking and distributed architecture focusing on performance and low latency.
If you have numbers for you applications you will be able to easily calculate how big your cluster must be to provide very fast access to your data in a consistent and predictable way.