What is the maximum number of controls that can be placed in a .aspx page ?

i don't think there is a max amount of controls you can have on a page, however a couple things you may want to consider.   How big is this going to make the page?  Are your users on dial-up?  Any of them reference a database, or the file-system?

All of these things can cause the page to timeout (if the timeout is not set), also, it may just load too slow for someone to stay interested in the page, potentially causing a loss of visitors.

Personally, I wouldn't throw 500 controls on one page.  I'd break it up to at least 5 pages....

There are no known limits for the number of controls on a aspx page. The system should be able to render 500 controls but the question is how usable is a form with 500 controls going to be.

precisely

This is for an intranet site, 189 are textboxes, 189 labels that indicate valid input and 49 labels to change state considering selection critera, a max of 160 will be enabled at a time.