Your event organizer will choose the amount of meeting slots for the conferences themselves. This means no two events will have the same amount of meeting slots.
As the event date approaches, other attendees will naturally fill up their meeting slots with meetings, so make sure you send meeting requests early and often!
Some attendees may also choose to block networking slots for any reason, meaning that you won't be able to meet with them during those times.
There can be many reasons why you don't see any available meeting slots when requesting a meeting from someone:
If you want to meet with someone who has no open meeting slots, the only course is to wait and hope they open up a meeting slot, either by cancelling or some other means.