Why are there so many vias on this board?

This is generally referred to as via stitching, and it's generally used to reduce either the high-frequency electrical impedance or the thermal resistance between layers. It can also be used to provide a low DC resistance path between layers for high current pathways. In this case the reason is certainly RF impedance, however the level of stitching shown is probably overkill even for a 900MHz RF part. However it's easy to do, and doesn't generally hurt anything on a board as sparsely populated as this one.

You would need to consult the design documents to determine the stackup details if the layers aren't clearly visible. Often for dev/eval boards the manufacturer will provide a full package of manufacturing documents.

It's a high-frequency RF part. 900MHz = 30cm wavelength. So even a board that's a few cm across is a significant proportion of a wavelength. The vias are to make sure that the top copper is really a ground plane, and not some weird unintended resonator.

I assume there is a copper pour on the top as well, and the bias are stitching the top and bottom planes together. Depending on the frequency of operation, it is possible that the via spacing would help to cancel out emissions. But in this case this effect would not be significant.

What I find interesting is the different via spacing and sizes in the input and output sections of the board. These must be significant, probably contributing to impedance coupling or simply filtering. I’d be curious to know the relation between via spacing and wavelength in those sections.

Of course, these could also be attachment points to simplify test setups. You might be able to get a straight answer in the manufacturer’s forum.

In low frequency boards, you would find prototyping sections that look very similar, but that is clearly not the purpose here.