It depends on your Trick or Treating Skills, leveling up lets you
scare more people. (The Creepy Doll obviously also increases the
number of users you can trick or treat.) I don't know exact numbers
but once you hit the limit, you're just told you can't trick or
treat today anymore (unless you level up). So I never cared to
count how many users I scare before I hit it because it doesn't
much matter to me.
You can see the time PH is running on at the bottom left of every
page, reset happens when the day changes.
As for no candies from interacting, I hear that's just a pesky
little bug Riako hasn't fixed yet because he's been getting his
beauty sleep.