How many slides can I add to the hero slider?

Really, you can add as many as you like but I would practice some constraints and keep the number of slides minimal.

The design concept of a hero slider is awesome. At the same time, the idea of packing a ton of valuable marketing messaging (slides) at the very top of the page is very enticing.

However, with Dwight's help, we're going to have to burst your bubble.

The fact of the matter (tons of UX research) is you'll be lucky if a visitor even sees your second slide.  We have all become accustomed to the "flick and scroll" way of browsing the web. This helps us know if we're in the right place and we care to dig into the content on the page.

For this very reason, I highly recommend keeping the number of slides to a minimum.  If there's important info that conveys the value of your product/service, consider positioning that content a little further down the page.