The 58-year-old got an interesting variety with little childrens notes, stickers and posters and she even was gifted a Prince Harry book.
"Things keep on coming and its from people all over the country. They have really personalised the cards, it is incredible."
Ms Rennie was blown away by the cards that had arrived in her home on Wednesday and was keen to respond to them all.
When she was just 15 years old living in Australia she made a pen pal in Christchurch and flew across the Tasman to visit her.
From there her passion spiralled and after being diagnosed with dementia and Alzheimer's she got a real kick out of writing to the royal family and Mick Jagger.
Every day she will sit down and just write to people.
On Saturday Ms Rennie received a letter addressed "from Prince Charles" and Ms Burns said her reaction was priceless.
"She was actually ecstatic, just so happy, it was amazing of the nurses to do that for my mum and it made me realise how much she loves receiving cards."
On her special day Ms Burns took her mother to get her nails done in Napier and treated her to her favourite things.
"She loves McDonald's, caramel chocolate and Coca Cola so I took her all of these things."
Ms Burns said she would help Ms Rennie write thank you cards to all those who made her mum's day.
"It was such a great response and I am so thankful to all those who sent cards and I am sure my mum will write back to you all."
