The second registered global variable is $argc which contains the number of elements in the $argv array (not the number of arguments passed to the script).
Like every shell application, the PHP binary accepts a number of arguments but your PHP script can also receive arguments. The number of arguments which can be passed to your script is not limited by PHP (the shell has a certain size limit in the number of characters which can be passed; usually you won't hit this limit). The arguments passed to your script are available in the global array $argv. The zero index always contains the script name (which is - in case the PHP code is coming from either standard input or from the command line switch -r). The second registered global variable is $argc which contains the number of elements in the $argv array (not the number of arguments passed to the script).