Are "crowd" and "evidence" countable or uncountable nouns? Can we use articles a/an with these: a crowd/ an evidence? I am not sure.
Crowd is a general noun, so it is uncountable. You don't use any articles with it.
Evidence is a countable noun, so you can use articles (both definite the and indefinite an) with it.
The way to know which nouns are countable or uncountable is to try using the nouns with a number.
Noun 'evidence' can be used with numbers. It is correct to say for example: The police found one evidence/three evidences in the crime scene.
It would be difficult to join uncountable noun (like crowd) with a number.
You usually don't count crowd. It is correct to say There were