A crowd is really defined as "a lot of people for the circumstance", not any particular number.
The OED says the noun comes from the verb, which has a sense of press, push, or hurry. So to be a crowd there has to be enough people they feel pushed together, either literally or figuratively.
Ten people in a small room is a crowd. Ten people in a large car park is not a crowd.
There's an expression "Two's company, three's a crowd", which means if two people want to be alone, three people feels very crowded.
Indeed, if just one other person stands too close, it would be usual to say "Don't crowd me!"
There are many derived meanings, including
- the audience of a sporting, musical, political etc event ("the crowd was hardly worth the name, only 10 people")
- the shoppers of a busy commercial street ("Fight the crowds on Oxford Street")
- "the people" or "society": ("He didn't have his own opinion, he just went with the crowd")
- group of associated people, OED: "noun. orig. U.S. A company; ‘set’, ‘lot’. colloquial." ("She went to see the chess playing crowd", which doesn't even necessitate that all those people are ever together.