In most cases (but not 100%), this is the limit of PHP, not WP.
You can check / configure it by looking at your php.ini.
Basically the problem is variable max_input_varsin php.ini.
This variable was introduced in PHP versions 5.3.9 and defaults to 1000.
You should try to increase it to 1500or even 2000.
: ( , )
php doc, , $_GET, $_POST $_COOKIE .
, save menu wordPress, POST, :
"Total POST variables" = 11 * "number of menu items" + 9
, , 90
( 11*90 ) + 9 = 999
1000, POST .
, POST Ajax ( ), - .
"Total POST variables" = 10 * "number of menu items" + 3
, - ( ). 10,0000 , ..)
PS:
googling , .