How hot the earth will be if we have 20% CO2 in the atmosphere instead of ~0.04? and how cold would it be if doesn't have any CO2?
The concentration of CO2 in the Earth's atmosphere has remained less than 1% in the past 470 million years when land plants first appeared--long before mammals, and long before humans graced the earth.
If we are talking about having a biosphere comparable to the one we know, we need to be talking about CO2 and global average temperatures that are comparable to levels during the relatively stable Holocene epoch of the past 11,000+ years.  Those are the conditions that support life, with the current overshoot beyond 350 ppm being brief (in terms of geological time).
As for 0% CO2 concentration in the Earth's atmosphere, that would remove warming that essential for life on the planet. The goal is to have CO2 levels that are stable and also at a level which provides the warming for which life in the biosphere is accustomed to--which is less than 350 and probably even less than 325 ppm.  An accepted scientific paper is Target Atmospheric CO2: Where should humanity aim? [https://arxiv.org/pdf/0804.1126.pdf].
This discussion should also switch from the concentration of CO2 in the atmosphere to global emissions from a few human activities (roughly 90% being combustion of fossil fuels). The question to ask is how much do those human emissions need to change for the current annual increases in CO2 concentrations to stop rising?  They need to return to emission levels that were essentially zero as they were prior to the Industrial Revolution.  That is when natural global emissions and absorption (aka sinks) cancelled one another out to keep levels stable.  However, cutting global human CO2 emissions by half (or let's say 60% to be safe) will stop the rise in CO2 levels for a few years.  If cuts do not go further within the decade toward 100%, CO2 levels in the atmosphere will start to rise again.  Reference: IPCC AR4 FAQ 10.3 [https://archive.ipcc.ch/publications_and_data/ar4/wg1/en/faq-10-3.html].