when I have to estimate my projects or CRs, I simply create basic Test scenarios (high level) ones and get an idea of how many test cases I might be working with and their complexities. Field or UI level test cases can be run and written at a pace of about 50-100 per day/per person. Medium complexity test cases (with 10 or more steps) can be written about 30 per day/per person. High complexity or end to end ones are at a rate of 8-10 per day/per person. All of this is an approximation and there are other factors such as contingencies, team’s proficiency, available time, etc. have to be taken into consideration but this has worked for me in most cases. So, for this question, this would be my answer.