"How many stops are there?"
Translation:Hvor mange holdeplasser er det?
Because 'der' refers to a place, eg: the cat is there (in that place) = katten er der.
'Det' means 'there' as in something that exists, eg: there is (a thing that exists) = det er en kat.
But in English those words are the same. So, for example, you could have the following exchange:
"We're going into town for a drink."
"How many pubs are there there?" (ie, how many pubs exist in that place)