Orla Galton, from Blackmill, Bridgend county, was diagnosed with leukaemia at three and blood transfusions helped save her life.
She endured them for years, but her mother, Sharon, said they were a lifeline.
The Welsh Blood Service wants 200,000 more donors to replenish stocks.
"When Orla was having chemotherapy her bloods hit so low that she became anaemic, she would have no white blood cells so she couldn't fight any infection," she said told BBC Radio Wales.
"It's quite a dangerous time as while their bloods are going down, they can't carry on with the treatment. Orla needed blood transfusions so she could keep having the chemotherapy."
Orla, who has now been given the all clear, paid tribute to the donors who helped her.
"I would just like to say thank you," she said.
"I can't thank you more."
The Welsh Blood Service said:
A 12-year-old cancer survivor has urged more people to give blood, as the number of new donors has dropped by 40% over the past decade.