What is the maximum capacity of MongoDB? Is it a fully scalable DB?
Yes MongoDB is fully scalable.
They have a laundry list of deployments over at MongoDB Production Deployments. MongoDB is trusted with some of the biggest of the "big data" web 2.0 sites such as foursquare, etc, etc. To give an example:
Wordnik stores its entire text corpus in MongoDB - 1.2TB of data in over 5 billion records.
However, when you get to sharding you will want to ensure you don't run your nodes at close to capacity (see: MongoDB CTO on Foursquare’s Scaling Issues)
Yes, MongoDB is designed from the ground up to be a scalable database. You can split your data across several hardware nodes, known as sharding, which is great for scaling both reads and writes.
You can also keep synchronised copies of your data across different nodes, known as replica sets, which is good for scaling reads and providing failover.
Scalable production deployments use a combination of both. A great resource for learning more about MongoDB scaling is this book, Scaling MongoDB by Kristina Chodorow