Ok, here we go. I would absolutely not be concerned with temperature IF that is allowed for the machines, same with humidity. Modern servers support up to 47 degree ambient temperature and even old ones are made from components that are mostly also used in tower servers and allow ambients above 30. Intel and Google studies show no bad things happening.
Most important is to check what he manufacturers allow.
The problem is secondary stuff, MOSTLY USV - Batteries may not like this temperature.
The other problem you may have is delusion about what temperature IS. The underground room may have a low temperature because it is shielded from sun etc., but that may not give you a VERY big amount of dissipation - so you may simply heat that up VERY fast the moment you actually emit heat in it. My own server room is in the basement of our offices - well, 1 meter over ground, 1.5 meters below. NICE cool room. 4 servers and it heats up. Why? Because it is mostly cool because it is isolated and the ground is cool, but at the same time the heat does not dissipate in hugh amounts. For 20 servers that LIKELY does not work - you will overheat the room without active cooling component, which costs have to be factored in.
In our case we are installing air conditioning now and will ramp the excess heat into the house heating system ;) We also removed all batteries from the room (external large USV installed) and will increase ambient temperatures in the server room to the 40 degree level as we replace servers. That is still way below what the manufacturer or our hardware allows (47 ambient) but it will make active cooling QUITE efficient - we never had that temperature outside, and cooling with a cooler outer side is more efficient.
If you NEED active cooling, then humidity is a non-issue anyway as air conditioning can remove humidity.
So, the main question is - how much heating do you create, and that is directly the power consumption of your servers. Assuming 200 watt per server you run about 4kw. That likely is above your dissipation capacity.
So, the whole thing is not as easy to answer. Anyone focusing on temperature here so far totally forgets that temperature is not absolute, it is a balance between emitted heat and dissipation to environment. And the current temperature may have no meaning the moment you add the servers. It is unlikely you can add 4kw energy to that room without getting into active cooling of some sort (and air replacement is active cooling unless you manage to create the airflow just by heat differential - possible, but that requires building the building in a very special way).