"The tourists travel into those cities, in which there are a lot of church."
Translation:Azokba a városokba utaznak a turisták, amelyekben sok templom van.
For those learning English churches has to be plural in english after a lot of
because churches are countable.
("a lot of water" is fine, for example; it does not have to be "a lot of waters")
Yes. Any number or quantity or similar things, however vague or specific, all require singular.