The sum number of neutrons and protons in one of the isotopes of hydrogen is :

The sum number of neutrons and protons in one of the isotopes of hydrogen is 6.

There are 3 isotopes of hydrogen, mainly. The sum of the number of numbers of the proton is as follows,

Sum of protons = 1+1+1

P=3

Sum of neutron = 0+1+2

N=3

Sum of neutron and proton,

P+N = 3+3

P+N=6

Hence, the sum number of neutrons and protons in one of the isotopes of hydrogen is 6.