Is it natural to omit adverbials of place with the construction there is/there are. For example:
Person 1: How many people are there in the office?
Person2: There are 20 people (in the office).
In this case, the fact that you're using "there is/are" doesn't really determine whether or not you can omit "in the office". You certainly can but that's mainly because "in the office" is already mentioned in the question. If the sentence isn't following a question, omitting "in the office" may cause a lack of information.
Bonus: Since "people" (the unit) is already mentioned in the question, you can also omit it.
There are 20 (people).