How do I work out how many staff my restaurant needs?

Start from forecast covers by daypart, not from a headcount. Divide the forecast by the covers-per-labour-hour your own operation achieves on well-run shifts to get the labour hours the week actually needs, then build the roster to place those hours where the covers land. Cross-check the result as a share of forecast revenue — labour typically holds around 25–30% of sales, though formats vary by design.

From How Many Staff Does a Restaurant Need? The Staffing Math — the full reading, with the method behind this answer.