There's no universal number of challenges to run at once — it depends on your concept.
A fine-dining or sit-down concept typically does best running a single challenge at a time (often a broader, overall "restaurant challenge") and watching how it performs before adding another. A QSR with a wider menu mix can often comfortably run two or three simultaneously, since each one can target a different, specific deficiency that shows up in your sales data.
The general guidance is crawl, walk, run: don't launch more challenges than your team can actually execute and market well. Start with one, prove it out, and phase in more from there. It also helps to distinguish two categories as you scale up — evergreen challenges that just run passively as a "pick your own adventure" option, versus limited-time event challenges that need a real marketing push and shouldn't be always-on.