There is no number of blog post needed to start your blog. As soon as you write your first post, you should publish it. Please don’t write a bunch of blog post and let it sit until you start your blog. Start posting your blog post as soon as you have them done.
The key is to produce content on a schedule. WordPress has a time stamp feature where you can write of many blog post you want and schedule your blog post out to the future date. So I would pick a schedule to post per week, write out all of your blog post then schedule them out during the week.
How many blog post should I write?
I try my best to do 3 post per week but I have changed the way I post my content on this blog. Now I will do one blog post per day. Now I will time stamp those blog post in the future. Write as much as you can write. The more you write, the better the traffic you will get but if you want to, you can write a blog post right now and publish the post after it’s written.