Foremost the number of times a gene is found does not directly relate to its expression, and mutant proteins are often significantly more reactive than their counterparts. Also having more genes means the risk of getting a damaged gene goes up.
Most importantly people for the last 20 years have tried to genetically integrate working p53 into cancer cells, and it has little effect on most types of cancer.
Some cancers (testicular if I recall), are the result of a p53 mutation. Crucially, fixing the mutated gene did not fix the cancer. The community I work with has a lot of skepticism about claims that simply increasing the quantity of tumor suppressor gene will fix cancer.
Most likely having more genes means that there was a higher chance of receiving a viable beneficial mutation somewhere early in the animals evolutionary history, although it does little for the individual animal in the present day.
Most importantly people for the last 20 years have tried to genetically integrate working p53 into cancer cells, and it has little effect on most types of cancer.