65535 is the number ........... 'WE' count from ZERO = 65536 in total it is also 2 to the 16th power
For a change I will translate into English..

the TOTAL number is 65536.. as Foxy commented 0 is counted as a port.. so counting would be like counting your fingers.. the first one is 1.. now count them with the first one being zero.. who has stolen the finger.?

Is there any application (useful or malicious) that uses Port 0?