generate consecutive number in post meta

hello,

a plugin question:
I want to generate for some post and pages a consecutive number (pinID) . Therefore I made a button in the area under the title, behind the permalinks and shortlinks buttons.

ok, button is there.
where should I save this value, I think postmeta should word, but how can I get the highest ID from all post/pages.

and how can I start the script to do these action, when the button is clicked, and after this the page should be displayed again.

the button is generated with jquery:
jQuery("#titlediv .inside #edit-slug-box").append('<a class="button button-small" href="????where to get the link????" target="_blank" title="Create the 4-digit PinCode for this page">CREATE PINCODE</a>');

regards