I think the "conquest" entails that as a species we are not immortal therefore we have to reproduce so I think what needs to be conquered is all other forms of life that pose a threat.

I think it is important to recognize the distinction between number and measure. It's hard to say exactly what number is, but measure is an application of number. When we apply number in the course of measurement, we make certain assumptions about the thing which is being measured, so there is a third thing which enters into the picture here, and this is a type of medium between number and measure. This is geometry. Geometry consists of assumptions which are utilized in applying number to measure.

"Conquest" here refers to the production of accurate geometrical assumptions. When our geometrical principles become accurate, we may "conquer" the world with understanding because this will produce accurate measurement.

Back then, 'conquest' was to be taken literally - everyone was in 'conquest mode', which, needless to say, was a wrong (and primitive) mode of thinking. Kings though it their 'duty' to conquer and seek the associated 'glory'.

Descartes was referring to understanding nature, which included understanding how we 'worked', which would enable us to live beyond the limits of nature - that is, to live forever, or at least "secure our consciousness against a harsh and deadly universe" (how i like to phrase it).