The King has been overwhelmed by messages of support from around 7,000 people around the world since his cancer diagnosis, Buckingham Palace says.
Many people have written to tell him about their own experiences with the disease or offer advice. One person told him: “Lift your head, stick out your chest, stay positive and don't let it get you down. Believe me, it works, but the main thing is family.”
One boy wrote: “Never give up. Be brave. Don't exceed your limits. Get well soon.”
Charles was particularly struck by a card showing a dog feeling sorry for himself after medical treatment, with the message: “At least you don't have to wear a cone!”
The collar, which prevents animals from aggravating a wound or stitches after surgery, is sometimes called a “cone of shame” by pet owners, or an “Elizabethan” or “e-collar.”
The king laughed when he found the card, Buckingham Palace's correspondence team said.
Supporters who have included an address will receive a thank you, the team said.
On Wednesday, the king carried out his first official face-to-face duties – an audience with Prime Minister Rishi Sunak and a meeting of the Privy Council – since his condition was made public.
Footage showed Mr Sunak saying: “It's wonderful to see you looking so good”, and the King replied: “It's all done with mirrors.”
The 75-year-old said the messages and cards of support he had received brought him to tears. The prime minister told him that “the country has your back.”
Since the diagnosis, Charles has postponed all his public duties, but continues to work on his red boxes of state documents.
Prince Harry made a whirlwind visit to see his father in London after the king broke the news of his diagnosis.
The Duke of Sussex spent almost as much time in the air as he did in England, on his first visit to his homeland since September.
The journey from the United States took Harry more than 10 hours, but his meeting with King Charles at Clarence House in London reportedly lasted less than an hour.
Harry later said his father's condition could “reunify” the royal family, fueling speculation that the prince could be allowed to return to the fold. He said he was grateful to be able to spend time with his father and reportedly suggested that he would be willing to temporarily return to his duties while the King is ill.
The King has been seen smiling and waving while attending church services at his Sandringham estate in Norfolk.
The type of cancer he has has not been revealed; It is also unknown how many rounds of treatment are planned, but so far she has received at least two.
Before his meeting with the prime minister, he held a meeting at Buckingham Palace of the privy council, whose attendees included the council's president, Penny Mordaunt.
At monthly council meetings, the King gives formal approval to orders that ministers have already approved.