Every day, multiple storylines start throughout the park. If no one joins them, they restart the next day. This includes "dead" hosts getting cleaned up and put back into place (like Maeve, for example).
Logan explains this to William when they first arrive in Sweetwater, and various hosts are trying to lure them into various storylines. We've seen this several times, like every day when Dolores dropped an item out of her pack for a kind stranger (guest) to pick up.
When a guest joins a host on a storyline - whether it was invented by the host or created by the guest (as in the case of The Man in Black dragging various hosts along his journey) - that host is no longer available for other guests, and the storyline that they typically start may be unavailable too. For example, Dolores is no longer in Sweetwater dropping the item out of her pack for a kind guest to retrieve for her, so that storyline is unavailable. But when new guests arrive, there are still a myriad of storylines they can follow after being lured in by all the hosts in Sweetwater.
My answer is based on having watched all the episodes, and considering / discarding other possible explanations like all the guests arriving at once, or presuming there are multiple copies of each host.