On the free buyer tier the cap is 2 active demands at once — and the word to notice is active, because the limit counts what is open right now, not what you have ever posted.
A demand counts as active while it is published and collecting offers. Anything you complete, close or cancel drops out of the count and frees the slot immediately, so you are never permanently out of room. Your current figure against the cap is on Settings › Usage under Plan limits; My Demands is where you close one to make space.
Two habits keep you under the cap without losing work. Close a demand once you have awarded it or decided against every offer — a finished demand left open is a wasted slot. And when you want to run a similar demand later, Duplicate (new demand) on the My Demands row rebuilds it from the old one rather than from scratch.
If two at a time is genuinely too few, a paid buyer plan raises the cap; upgrades take effect immediately from Plans & pricing.
Also asked as: What is the active demand limit? · Can I run more than one demand at the same time? · Does a closed demand still count against my limit? · Is the number of demands capped?