Dennis @ PerfectForms
I recommend building a very simple form to contain the number of registrations. Then build a connection to that form. In your registration form build a behavior when it opens that connects and brings back the number. Compare that number to your target number (held in a hidden field in your registration form) and if it is equal or less than the target, proceed. If it is higher then show a message and disable the submit button.
When the form is submitted use the connection to add a 1 to the number in the form with the count. You may even want to check the number of registrations one last time to make sure that another user did not take the final spot in the time that it took to fill out the form.
Here is a link to the page training videos on our web site. There are two videos on connections : http://www.perfectforms.com/training.php.