Actually zero.... if all humans die today, the climate will reverse itself within 10,000 years.

But with humanity 'thriving' we will need to reverse climate change with technology, within a capitalist system, as an ongoing process (as long as there are humans). AI, quantum computing and nuclear fusion breakthroughs are soon to be upon us, optimistically we will reach the point where we are able scale them commercially to where the marginal cost of intelligence and energy is close to zero. Economically this means that there will be infinite resources able to be diverted towards engineering a better environment and uplifting humanity.

Sorry i digress and ramble, there is no number here to answer your question based on current technology and economics.