<p>As far as Google searches tell me, the maximum allowed number of cookies depends very much on the browser, however I cannot find any recent data as to how much cookies are allowed on modern browsers.</p>

<p>I need to store a somewhat large number of user preferences in cookies (for not-yet-registered users), so what would be the best way of doing that? (Also, those cookies would be accessed both via javascript client-side and php server-side) </p>

The best way would be to not store them in a cookie at all.
Store them in a database, and store the DB key in the cookie.  If it's just a few preferences then security isn't much of an issue.
Don't forget that cookies will be sent with every request - if you have 2kb of cookie data and load 10 images on a page, that's an extra 22kb of data.