I don't know that there is a actual limit, but as a practical matter you
might want to show some discretion. Remember that each will add
significantly to the time it takes for that page to load. Your viewers came
to your site for information...not to wait around for ads to load.
When I was trying out a JavaScript for time, day and date, I attempted to
put more than one on the webpage. When accessing the webpage from the IP,
it showed only the first time, day and date. 'Don't know why. To me, just
another mystery of the electronic age.
