You need to have a bargaining group of >=2 in the US per the NLRA. But some unions may require more people in order to represent you. And not all businesses are subject to NLRB jusisdiction and some types of employees can’t unionize.

https://en.wikipedia.org/wiki/Bargaining_unit

https://beta.dol.gov/policy-regulations/unions-collective-bargaining/what-union/forming-union

https://www.nlrb.gov/about-nlrb/rights-we-protect/the-law/jurisdictional-standards

FMHY - not exactly a megathread, but kinda