I have just submitted my thesis last month for review and I am actively applying for postdocs. My PhD research was on biomass conversion to chemicals. I have e mailed to 15-20 profs (different cv and resume to each of them according to their research area) and also applied to some open positions but I haven't received any positive response from them. My PhD mentor says I need to apply to 100-150 people. Is this true?

Unfortunately sounds true. Getting a Postdoc position is more difficult than getting a PhD position because the funding for Postdoc is limited. Moreover they expect you to apply for funding and bring them money. You should also consider applying in industry. You are now in a strange state where you are over qualifified in sense of academic degree and education and under qualified in sense of lack of industrial experience. I think it is the porblem of most PhD graduates. Just try again and do not get disappointed.

I haven't tried finding a post-doc yet but I thought the best way to find a post-doc is networking. Most of of the post-docs I know, knew the post-doc supervisor in some way beforehand. Either via recommendation or meeting a conference. I know my supervisors don't advertise post-docs and if they don't know anyone, they reach out to their network for a recommendation. I know this isn't useful but maybe reach out through any network you have or your supervisors.