I only have one patron so far, so I may not be the best person to help with this, but I don’t think you necessarily need to have a backlog of posts before you launch. It is good though to have some sort of a schedule for adding new content so that people keep coming back.
How many posts per week varies greatly creator to creator based on what you do and other, outside, time constraints. I’ve seen people that post only a few times a month and others that post nearly every day. I’ve been aiming at 3-5 posts a week, but I’m thinking I may scale that back to 2-3 posts to give each post more time to get seen before getting buried under new ones. One of those posts is my Friday ‘weekly update’ post where I can give progress reports on all current projects, new ones I’m considering, and just plain life sometimes. Whatever you decide, it needs to be sustainable for you. Something that you can easily keep up with without running the risk of burning out.
You might also consider having some public content so people can get a better idea of what it is that you do that they might be interested in and want to support. Maybe use the early access feature where you patrons see the posts first, but, eventually they become public (I usually do this with my finished pieces, but leave any work-in-progress/behind the scenes posts as patron-only). It can be a good idea to, first, build up you Patreon followers and then try to convert them over to patrons. People that follow your public Patreon content are probably more likely to become a patron than those that only follow you on other sites since they already use Patreon.
Remember, content doesn’t have to be all of finished work. We all work at different paces depending on what we do and may not be able to have a lot of finished work to show per month. However, posting concept sketches, process posts, etc. is also all content that potential patrons may be interested in and keeps you page regularly updated between finished work.
Good luck! I hope some of this is helpful to you!