ISO 27001 gives no number. Clause 9.2 asks for internal audits at planned intervals and for an audit programme that sets the frequency. In practice almost every certified company audits once a year, because the certification body comes once a year.
The certificate runs on a three-year cycle: the certification audit in year one, a surveillance audit in each of years two and three, then recertification. Each external visit expects an internal audit carried out since the previous one. Six to ten weeks before the external date works well. The evidence is recent, and there is time to close findings and hold the management review in between.
After the first year, one audit does not have to cover everything. Your programme can split the standard over the three years, for example all clauses every year plus a third of the Annex A controls each time. By the end of the cycle every applicable control has to have been audited at least once.
Some events justify an extra audit outside the yearly rhythm: a move to a new cloud provider, an acquisition, a new product that handles personal data, or a serious security incident. A short audit of the changed area is enough in those cases.