Thank you Diego for your question, indeed it is a common mistake! Technically you can’t. The reason is that people is plural in English. We say ‘people are’ and ‘people were’.
So you have to say:
There were many people. OR There were not many people.
As for the use of ‘there is versus ‘it is’, remember:
It is important to remember the difference between ‘it is’ and ‘there is’.
What is that on the roof? It is a Frisbee. – Es un Frisbee.
Is there a Frisbee in your trunk? No, there isn’t one in my trunk. – No, no hay uno en me maletero.
Remember that hay in Spanish has two forms in English:
There is a carton of milk and there is some butter in the fridge.
There is – singular and uncountable nouns.
There are three people in the waiting room.
There are – plural nouns.
To ask the question, we invert ‘there’ and ‘to be’:
There aren’t any Post-its in my office.
Are there any Post-its in the cupboard?
In the past, we use ‘there was’ and ‘there were’:
There was a parade in the city center yesterday.
There were a lot of people on the main street.
I hope this helps you. If you want to review the use of There was/were more, please refer to our lesson: Curso Inglés gratis – There was/there were, there will be
as well as the following videos:
English Grammar: past «there was/were = hubo, había»