The fluorescence excitation spectra show the change in fluorescence intensity as a function of the wavelength of the excitation light. Does that mean, that at the wavelength of excitation maximum the most of the molecules get excited? Or do they just emit more photons per molecule per unit time?
You have to understand how the excitation spectrum is collected. You have to know (or at least have an estimate) of the emission maximum of a given molecule. For example, if we have quinine, I should know that it emits blue light and maximum wavelength of emission is ~ 450 nm.
Now we will fix the emission wavelength at 450 nm and scan the entire UV-VIS range for excitation. Say, we would excite quinine with 200 to 700 nm. One wavelength at a time, and count how many photons corresponding to 450 nm were emitted. This plot is called the excitation spectrum.
An excitation spectrum basically tells you which wavelength (from a given instrument) is going to show the highest intensity. I emphasized "a given instrument" because an excitation spectrum has to be corrected for the light source intensity. It will vary from instrument to instrument. Nobody corrects it because it is a long procedure as unfortunately, normalization cannot correct it.
Anyway, a corrected excitation spectrum should match in shape with the absorption spectrum provided that there is only one fluorophore in the molecule.
I wanted to connect the excitation spectrum with an absorption spectrum because we can now connect the molar absorptivity with the excitation. Consider molar absorptivity as a measure of probability of excitation. Higher the value of molar absorptivity, more molecules will absorb the light and hence more will emit. So your first interpretation is correct. It is a collective behavior of the all the molecules which are being illuminated in the cuvet.
Also remember that excitation emission is normally a one photon excitation and one photon emission in majority of cases until and unless you are using fancy lasers. So you cannot have one photon in, and multiple photons out in routine cases. In rarer cases, you can indeed have two photon in for excitation and one photon out. You need lasers again.