What formula or worksheet application would allow me to estimate how much sales is required to how a given number of employees?

Far, far, far too many variables.
In the US, a huge overhead is required for half the SS, unemployment insurance, workmen's comp, and liability. Comp varies a lot, by how risky the job is. Then there's health insurance, in some cases. My state is raising the minimum wage each year. I could go on all day.
Having run offices for small proprietors, I can tell you that starting to be an employer (which also requires a federal and state EIN) is a tremendous amount of work, knowledge, filing of reports, and money.
There is NO app for it.

I'm not even an accountant and I know you need more information than that.

The breakeven formula can be adapted to provide the answer. Basically staff costs are fixed costs, however there may be other costs involved. In simple terms; divide the cost by the gross margin percentage

What is the percent of profit, for total sales?
What percent of total profit is allowed for employee costs, including salary plus employee overhead costs.

That is a individual company issue, since they all have different percents of overhead for employee costs, (insurances, government taxes, and more) and have a different idea of what percent of profit can beused for employees.

I wrote a long response yesterday and it disappeared.
There is no such worksheet. Far too complicated. Even something like Workmen's Comp can be twice the amount in one risk category as another. In the US, there are variations by state. I could sit here all day writing about it. You have to do your own work on your particular situation.