It’s a statistical measure comparing the test taker against the average, much like percentiles.
At a certain IQ score, somewhere in the 170’s I think, the expected number of individuals with that IQ is about 1.
If we had absolute measures of intelligence (that would be a breakthrough for the ages), then we could say “A is twice as smart as B” and award A twice the points of B. In such a system, the sky is the limit for the number of points.
EDIT: If/when we build a human-level AI, perhaps we could use the number of transistors / artificial neurons involved as a proxy for an absolute measure of how difficult it is to answer some problems. This would be imperfect but better than nothing.