Does every wavenumber of IR result in a different kind of vibration? If that is true, what if a molecule absorb 2 different wavenumbers (which cause different rocking and symmetrical stretching for example ) in the same time?
In most cases, one can expect different vibrational modes to have different energies, but that is not always the case. Two or more modes may be degenerate, i.e. have the same energy, although they do not represent the same vibrational motion. This often follows from the symmetry of the molecule and can be deduced from group theory. For example, in ammonia, there are two cases of two-fold degeneracies (see on this page).
As to the second part of your question, molecules may indeed absorb radiation that does excite two (or even more) different vibrations at the same time. The spectral signal you get from such an absorption is called a combination band. Similarly, you can also excite the same vibrational mode with two quanta, wich is called an overtone. The energy of the radiation that is absorbed is in both cases roughly equal to the sum of each individual excitation ("roughly", because anharmonicity becomes important here).