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.