Israel had about 1 Levite per 11 other Israelites. But that’s not particularly helpful.
This is a difficult question to answer because really… Who knows!?! The best you can do is have what you think is an appropriate ratio. But that then raises the question… Not only do you need to define whether “staff” means pastoral or also includes admin staff, but even trickier… What defines a “person” that your doing the ratio to?
See, imagine a church that has 1 paid staff. They might have 50 adults on average each Sunday. That’s 1:50 ratio. But there’s also 20 kids… So that’s a ratio of 1:70. And then there’s 20 or so people who come along irregularly. So that’s 1:90.
Sounds like good value, right? Wrong. A church’s goal shouldn’t be to get good value out of their staff – as if they are a 1900s factory worker you need to squeeze the most productivity from. You don’t want lazy staff, but that attitude is part of the reason why pastors burn out. Their church expects them to take responsibility for 90 people, and then reach the 10,000 other people in their suburb! That’s a ratio of 1:10090.
Rather than, “How many people can we spread our staff across?”, the better question should be, “How many staff can we get for all the lost people in our city?!”