It’s one of the simplest rules in English grammar - use ‘much’ when the noun is uncountable and use ‘many’ when the noun is countable.
So why do so MANY people have so MUCH trouble with these terms. Simply put, English speaking and writing is not about remembering rules, it is about habit. So if you get into the wrong habit when using ‘much’ and ‘many’ you can go a lifetime repeating the mistakes.
This app contains over 600 sentences. Learn to recognise when ‘much’ is correct and when ‘many’ is correct. Also learn common adjectives which support the use of these terms such as ‘so’, ‘too’ and ‘very’ as well as common English sentence structures.
Practice with the app regularly when you have spare time and you will start to remember the correct use of ‘much’ and ‘many’ without trying.