The fact that water has two hydrogens attached to a single oxygen is determined by the number of bonds that an oxygen atom can form. As you may know, all atoms are comprised of a small nucleus surrounded by 'cloud' of electrons. While these electrons don't orbit the nucleus like the planets orbit the sun, they do tend to spend more time at certain distances from that central spot. These distances depend on the energy of the electrons (how 'fast' they're travelling around the nucleus). The most energetic, so furthest away, electrons are called "valence" electrons and these are the electrons that interact with other atoms. An oxygen atom has 6 valence electrons. As it turns out, it doesn't cost much energy to add up to 8 electrons to an atom like oxygen's valence shell, but adding a 9th electron will cost a great of energy (you'll learn why if you study chemistry/physics in college) so the oxygen atom has space for two more electrons before they get very hard to add. The hydrogen atoms provide these electrons when water forms. Adding more hydrogen on top of H₂O would require those extra electrons from hydrogen to occupy much higher energy levels than is typically available. If H₄O were ever to form, it would spontaneously decompose into H₂ + H₂O because the energy of an H₂ molecule is much lower than those extra two O-H bonds.

The proton has a +1 charge, oxygen has -2. Hence to make neutral molecule H₂O you need 2x(+2) +1x(-2)=0

Atoms are made up a nucleus, which contains protons and neutrons, as well as electrons, which form shells around this nucleus. In each electron shell, there are different numbers of electrons. The first shell can only fit two electrons, and the second shell fits eight. Oxygen, number 8 in the periodic table, has eight electrons, 2 in the first shell and 6 in the second. Hydrogen only has one electron which is in the first shell.

Oxygen and hydrogen share one electron when they bond. By bonding with two hydrogen atoms, oxygen has its 6 outer shell electrons + 2 shared electrons from the hydrogen making 8 (and a filled shell makes a happy atom. Each of the hydrogen atoms has their 1 electron + 1 shared electron to fill their shell with 2 electrons.

If oxygen bonds with 4 hydrogen atoms, it would have too many electrons to fit into its outer shell.

Oxygen needs two electrons to fill its outermost electron shell. Hydrogen has one electron to play with. Therefore, an oxygen atom needs two hydrogens to bond to in order to complete its shell.

Oxygen atoms (O) try to find 2 electrons on other atoms that the other atoms will share with them. So 2 hydrogen atoms (H) each share their 1 electron with 1 oxygen atom. That makes 1 molecule of water, H₂O.

Different kinds of atoms try to share different numbers of electrons with other atoms. Some kinds of atoms have electrons to share, and other kinds of atoms are trying to find more electrons. So when 1 carbon atom (C) makes 1 molecule with hydrogen atoms, it needs 4 hydrogen atoms, and the molecule is CH₄, which is methane.

In the same way that you have two hands and generally can hold two things, oxygen tends to favor bonding with two things because of the energy of its electrons. However, depending on the conditions, oxygen can bond to three hydrogens, which happens where there are many hydrogens (that is, something is strongly acidic).