What should one do if they lose count of their tavaf circuits?

If a person is uncertain about the number of circuits completed, they should base their count on the lesser number about which they are certain. This is the view of the majority of scholars. Sheikh Ibn Uthaymeen said: 'Build upon certainty, which is the lesser number.' For example, if uncertain whether 5 or 6 circuits were completed, assume 5 and complete 2 more. Sheikh Ibn Baz added: 'The principle in Islamic jurisprudence is that certainty is not overridden by doubt' (Fatawa Ibn Baz, vol. 16).