We use many to refer to a large number of something countable. We most commonly use it in questions and in negative sentences:
- Were there many children at the party?
- I don’t have many relatives. We’re a small family.
We don’t normally use many alone before a noun in an affirmative statement:
There were a lot of people at the swimming pool this morning.
Not: There were many people …
Cambridge Dictionary
Many grammar books at intermediate level will have learners believe that "many" is preferred in question forms and in negatives.
“How many people are in front of that building?”
“There aren't many.”
But a native speaker could also respond with the more informal
“There's some” and “Not a lot.”
Arguing that “some” is the best choice because the noun building is preceded by the indefinite article is mistaken. Here are some examples with many in affirmative sentences:
Look, there's a car park (US parking lot) with many parking spaces.
Here's a book with many black and white photos.
I want a house with many rooms.
If the number of available parking spots, photos, or rooms were fewer then the determiner some would make a better fit.
So the answer is both determiners are grammatical, which one depends on the information the speaker/writer wants to convey. In everyday conversation, saying a building with some people outside is more natural because…
A lot of' or 'lots of', say, would be used instead of 'many' here in conversation. @Edwin Ashworth
See also: Is 'many' used in positive sentences or not?