If you get a Too many connections error when you try to connect to the mysqld server, this means that all available connections are in use by other clients.

The number of connections allowed is controlled by the max_connections system variable. Beginning with MySQL 5.1.15, its default value is 151 to improve performance when MySQL is used with the Apache Web server. (Previously, the default was 100.) If you need to support more connections, you should restart mysqld with a larger value for this variable.

MySQL Enterprise.  Subscribers to the MySQL Network Monitoring and Advisory Service receive advice on dynamically configuring the max_connections variable — avoiding failed connection attempts. For more information see, http://www.mysql.com/products/enterprise/advisors.html.

mysqld actually allows max_connections+1 clients to connect. The extra connection is reserved for use by accounts that have the SUPER privilege. By granting the SUPER privilege to administrators and not to normal users (who should not need it), an administrator can connect to the server and use SHOW PROCESSLIST to diagnose problems even if the maximum number of unprivileged clients are connected. See 項12.5.4.24. 「SHOW PROCESSLIST 構文」.

The maximum number of connections MySQL can support depends on the quality of the thread library on a given platform. Linux or Solaris should be able to support 500-1000 simultaneous connections, depending on how much RAM you have and what your clients are doing. Static Linux binaries provided by MySQL AB can support up to 4000 connections.