You'll hate this answer: It depends.
It depends on what you're selling, what your budget is, how well defined your audience is, how engaged you are with that following.
There is no set rule. Everyone has different success (and failure) and I would be leary of anyone who has a "set in stone" number for this. It is entirely dependant to what I outlined above.
If you'd like more help, or have other questions, feel free to send me a message or book a call.