They are joining in this role Queen Camilla, Prince William, Prince Harry, Prince Andrew and Princess Beatrice.
The Act was speedily discussed by the Lords and the Commons over the past few weeks.
With a larger number of people available who can stand in for the monarch, Charles can rest assured he will always have at hand at least two senior royals who can step in for him.
Before the approval of this Act, the role of Counselors of State was regulated by the Regency Acts 1937 and 1953.
These stated the monarch’s spouse and the next four people in the line of succession – no matter their status within the Firm – aged over the age of 21 held the title of Counsellor and could be chosen to deputise for the sovereign.
The new Act doesn’t exclude the Dukes of York and Sussex and Princess Beatrice – who are all non-working members of the Royal Family – from the role.
However, they are now highly unlikely to ever be drafted as, during a reading of the Counsellors of State Bill held last month in the House of Lords, the Lord Privy Seal Lord True said the Royal Household had confirmed that only working members of the Firm would be called upon to act in lieu of the King if the Earl of Wessex and the Princess Royal were added to the pool of Counsellors.
During that same reading, Labour peer Lord Berkeley brought forward an amendment that would have taken off the list of Counsellors the Dukes of York and Sussex if approved.
READ MORE: Meghan faces growing US backlash as Netflix PR strategy backfires
The peer said the question of who was eligible to act for the King needed greater “transparency”.
Labour peer Viscount Stansgate welcomed the increase in Counsellors.
However, he also described the addition of two specific royals as a “quick fix” and suggested a longer-term view was needed for deciding who should be appointed.
Among the duties Counsellors can be drafted to carry out are the signing of official documents, holding Privy Council meetings and receiving the credentials of new ambassadors to the UK.
At least two Counsellors of State at the time are needed to carry out duties on behalf of the King.
In May, the late Queen Elizabeth II issued Letters Patent to draft Prince William and King Charles, then the Prince of Wales, and have them attend the State Opening of Parliament on her behalf as she was suffering from mobility issues.
As King Charles still carries out visits abroad, he will likely need to rely quite regularly on stands-in.
The late Queen stopped undertaking foreign visits in November 2015, when she paid one last visit to Malta.