Is 52 a typo or did he really say that? The US has 50 states, so why not 51?
Probably a reference to the "joke" that Canada would soon become the 51st state
Maybe his views are more heterodox and he was counting Puerto Rico as 51st! (sarcasm, in case this isn't clear)
Greenland is the 51 state.
I suspect that the implication in context was that the 51st state would be Greenland. Which doesn't really help make this less of a diplomatic faux pas.