The answer isn’t a number really, it is a word: consistently!
Just keep on showing up and it will work. Be consistent in your marketing, plan your posts and perhaps schedule them or at least pre-write them so you are not stuck with what to say. Of course, ad-hoc posting is still great, and you need to be engaging with the people who like or comment on your posts as well, but it will ensure you maintain a presence if you pre-plan your content ahead of time.
Personally, I post on my four main platforms (Twitter, LinkedIn, Instagram and Facebook) at least once a day. In addition to this I use Instagram and Facebook stories most days too. I plan and schedule it all once a week, then the pressure is off, and I know even if I have a crazy day I will have posted everywhere.
Consistency is also about being on brand. Be authentic, whether that is you if you are a sole trader or authentic to your business brand if you are bigger. This is vital, because remember the point of social media is to get people not just aware of you and what you offer, but to get them to know, like and trust you as well. By being yourself, adding some personality and uniqueness to your posts, you will stand out and attract the right audience.
It is all about the long game, being consistent, keeping showing up and it WILL work. Trust me, I’ve tried it.