Question

What is the maximum number of connections for a SQLite3 database?

Why can’t I use it for very big websites, for example with 3 million users?

Answer 1

http://sqlite.org/whentouse.html explains “Situations Where Another RDBMS May Work Better”:

SQLite uses reader/writer locks on the entire database file. That means if any process is reading from any part of the database, all other processes are prevented from writing any other part of the database. Similarly, if any one process is writing to the database, all other processes are prevented from reading any other part of the database. For many situations, this is not a problem. Each application does its database work quickly and moves on, and no lock lasts for more than a few dozen milliseconds. But there are some applications that require more concurrency, and those applications may need to seek a different solution.