toes

I think the red number is the level you must be to enter that world, so 1 for liberty port as it is the first world, 15 for elf city as you can access it at lv 15. the time the monsters spawn in the would can be worked out by comparing the game time to your time. the game time is shown just under your battle rank. If you  go to the place where the other players are at that time then you will likely be in the right place to find them.