Auto increment in phpmyadmin

There are possible steps to enable auto increment for a column. I guess the phpMyAdmin version is 3.5.5 but not sure.

Click on Table > Structure tab > Under Action Click Primary (set as primary),click on Change on the pop-up window, scroll left and check A_I. Also make sure you have selected None for Default

In phpMyAdmin, navigate to the table in question and click the "Operations" tab. On the left under Table Options you will be allowed to set the current AUTO_INCREMENT value.

Just run a simple MySQL query and set the auto increment number to whatever you want.

ALTER TABLE `table_name` AUTO_INCREMENT=10000

In terms of a maximum, as far as I am aware there is not one, nor is there any way to limit such number.

It is perfectly safe, and common practice to set an id number as a primiary key, auto incrementing int. There are alternatives such as using PHP to generate membership numbers for you in a specific format and then checking the number does not exist prior to inserting, however for me personally I'd go with the primary id auto_inc value.