how many copies of tp53 (p53) gene do we have?
Humans have one copy of the p53 gene per set of chromosomes along with most other mammals (a notable exception being elephants with 20)[1]. Since it is located on an autosome (chromosome 17), each parent gives one copy of the gene[2]. Meaning two copies per human cell (and 40 per elephant cell). 



A little extra information on p53 and the effects of only having 2 copies:

Since humans only have two copies per cell and p53 forms a homo-tetramer, it only takes one mutated copy to lose functionality.[3] This is because p53 needs to tetramerize to function, meaning only 1 out of 16 p53 proteins are effective if one copy is mutated (24 possibilities and only 1 combination has 0 mutated copies, ~6% are functional). This makes p53 a rare human tumor-suppressor gene with dominant negative mutations.

But with elephants, the more copies they have, the less and less one mutation affects the amount of functional p53. ($\bigl( \frac {39}{40}\bigr)$4 probability of having a functional tetramer = 90% functional, which is very high)