I would like to create a database for each customer. But before, I would like to know how many databases can be created in a single instance of MongoDB ?
By default, you can run some 12,000 collections in a single instance of MongoDB( that is, if each collection also has 1 index).
If you want to create more number of collections, then use --nssize when you run mongod process. You can see this link for more details:
http://www.mongodb.org/display/DOCS/Using+a+Large+Number+of+Collections