Apps differ by orders of magnitude in weight, so counting them tells you almost nothing.
App count is a poor proxy for cost, because one badly-built app can outweigh ten well-built ones.
The measure that matters is what your storefront actually requests, which DevTools shows in about ten minutes.
Uninstalled apps frequently leave code behind, so stores carry weight from tools they stopped paying for.
Apps loading globally to support a feature used on a few templates are the most common and most fixable waste.
There is no useful number, because apps vary enormously. A store with twenty lightweight, well-built apps can be faster than one with five heavy ones.
The full guide has the formats, the deadlines and the worked examples.