Is there a specific number of fruit that you are supposed to eat on Tu B'Shvat?
The language of the Magen Avraham 131:16 is leharbos, which is specifically not specific.
I spoke to an elderly Jew who grew up in pre-war Poland who said the custom in his town was that only the wealthy could afford fruit on Tu b'Shvat and the one fruit of choice was a pineapple which he said cost a week's salary. So at least in some places, the answer was one. I personally haven't seen this custom as common today.
According to here:
It is said in the name of Rabbi Chaim Vital that one should endeavor to eat thirty different types of fruit on Tu Bishvat: ten different fruits which are eaten in their entirety, ten fruits of which only the interior of the fruit is eaten, and ten fruits in which only the exterior is eaten. Other kabbalists teach that only 15 different fruits are necessary.