343 Industries has assured fans that Halo: Infinite can be considered ‘Halo 6’, rather than a prequel or spinoff that the non-numbered title may suggest.
343’s Jeff Easterling confirmed that the upcoming Halo: Infinite is indeed the next main instalment in the series, despite not being a numbered entry. “It is Halo 6. You should just consider it Halo 6,” Easterling said during a Mixer Stream, via GameSpot (at the 1:22:20 mark).
Source: IGN.com Halo: Infinite Is Definitely Halo 6, Not a Spinoff or Prequel