How many parties are needed in any democratic system to compete in elections and provide a fair chance for the competing parties to come to power?

More than one party is needed for democratic elections. Two-party system and multi-party system gives a fair chance to competing parties.

Note:

Party system evolves differently in each country. The system that suits one country may not suit another. But one-party system is not considered fair enough because no other party get a chance to come to power. Hence, two-party system like in USA and multi-party system like in India is better for providing a fair chance to parties.