The rules involving election recounts vary from state to state. In some, like Pennsylvania, a recount is automatically triggered if the final margin reaches a certain percentage. In Georgia a certain margin is required in order to request a recount. In other states, like Nevada, no margin is required but there is a deadline for a campaign to request a recount.
2020 presidential election: How recounts happen in battleground states. See story here. https://abc7chicago.com/politics/recount-rules-what-happens-if-race-is-too-close-in-key-states-/7703352/