You should apply for business visa if you have the Invitation Letter from the organizer of the trade show, I think the original Notite Sheet from the authorized government unit is also required for Cameroonians, the origanior can apply for the notice sheet from Foreign Affairs Office or Committee of Foreign Econimic Relations and Trade. As for the stay duration, the visa officer will make a decision according to the invitation letter and the original notice sheet because they will prescribe the entry numbers, stay duration, etc. You can download the visa application form from this website https://www.travelchinaguide.com/embassy/pdf/visa-application.pdf