There have been four actors who have officially portrayed Albus Dumbledore in the main Harry Potter universe and related productions up to now. Direct answer

• Richard Harris
• Michael Gambon
• Jude Law
• John Lithgow (not in the main films, but associated with Dumbledore in later announced projects; if you’re counting all on-screen portrayals across media, some discussions include him in future/expanded adaptations)

Notes

• In the core film series, the role was played by Richard Harris in the first two films after which he passed away, and Michael Gambon then portrayed Dumbledore in the remaining films (six total). Jude Law later played a younger Dumbledore in the Fantastic Beasts film series, which is a separate spin-off timeline.
• If you’re focusing strictly on the eight main Harry Potter films, there were two actors who officially played Dumbledore: Richard Harris (Philosopher’s Stone and Chamber of Secrets) and Michael Gambon (the rest). Jude Law is counted when including the Fantastic Beasts films. Some discussions and fan resources also note other actors in stage productions or planned/rumored TV adaptations, but those are outside the core film canon.