This is an interesting question.. Check out this info from Statista… It is dated, but relevant.
How many are used is a matter of perspective – defined by what you consider as ‘usage’. Some apps are used daily – like social / collaboration apps. Some are used occasionally – less than once in a day / week / month. Spreading the limits to being used once in a year – I guess all the apps listed in the stores would be countable.
Curious: What is the average number of resident apps in a mobile device?