Atomic number: 1
Electron configuration 1s1.
If hydrogen gains 1 electron it will form the ionn H- . This is feasible and likely to happen because with on additional electron the electron configuration will be 1s2, and it will complete the las shell (same confiugration of He) which is a stable confirguration.
Then the answer is 1-. But you will find that H can also loose its electron and form the ion H+.
A more representative analysis can be done with Oxygen, whose atomic number is 8 and the electron configuration is 2s2 2p6, then by gaining two electrons it will acquire the stable electron configuration of Ne: 2s2 2p8
By gaining two electrons, the ion has two negative charges, this is O 2-.
Whith this I have ilustrated the method: 1) use the atomic number to make the electron confirguration, 2) look at the valence electron shell and determine if it is easier to loose electrons or to gain electrons (gain 1 or two electrons is easier than loosing 7 or 6 electrons) to acquire the electron configuration of the closest Noble gas (full valence electron shell)