The Russian language usually uses the genitive case to express partialness. However, some Russian mass nouns have developed a distinct partitive case, also referred to as the "second genitive case". In modern Russian, use of the partitive case is often facultative. In many situations, the partitive and the genitive can be used almost synonymously: чашка чаю (partitive) and чашка чая (genitive) both mean "a cup of tea". The partitive variant is preferred with verbs: выпить чаю, "to drink some tea". The genitive variant is used more frequently when the mass noun is modified by an adjective: чашка горячего чая, "a cup of hot tea". Check here for the source and more information

Basically, Много народу refers to another case, which is called partitive and it was used in older times. Since it is not used so frequently in a modern Russian language, this case is often merged with genitive and hence refers as the second genetive case. If you would read classical Russian literature, you would certainly encounter partitive, but in modern Russian you would mostly hear it colloquially.

Except for the partitive, there are also other cases in modern language, which are often used colloquially, but not given in official grammar books when teaching pupils in Russian schools. For example, if you ever listened to a regular conversation between Russians, you must have encountered a vocative case without even realizing it: Даш, ты меня слышишь? Нет, Саш.

Here at the end you can find more information about additional cases which are used in Russian nowadays. If you are simply trying to learn Russian, do not focus on them too much, in the end they constitute a small group of all the situations and will come intuitively simply by mimicking Russians/ learning some constructed phrases things by heart.

The Russian language usually uses the genitive case to express partialness. However, some Russian mass nouns have developed a distinct partitive case, also referred to as the "second genitive case". In modern Russian, use of the partitive case is often facultative. In many situations, the partitive and the genitive can be used almost synonymously: чашка чаю (partitive) and чашка чая (genitive) both mean "a cup of tea". The partitive variant is preferred with verbs: выпить чаю, "to drink some tea". The genitive variant is used more frequently when the mass noun is modified by an adjective: чашка горячего чая, "a cup of hot tea". Check here for the source and more information

Basically, Много народу refers to another case, which is called partitive and it was used in older times. Since it is not used so frequently in a modern Russian language, this case is often merged with genitive and hence refers as the second genetive case. If you would read classical Russian literature, you would certainly encounter partitive, but in modern Russian you would mostly hear it colloquially.

Except for the partitive, there are also other cases in modern language, which are often used colloquially, but not given in official grammar books when teaching pupils in Russian schools. For example, if you ever listened to a regular conversation between Russians, you must have encountered a vocative case without even realizing it: Даш, ты меня слышишь? Нет, Саш.

Here at the end you can find more information about additional cases which are used in Russian nowadays. If you are simply trying to learn Russian, do not focus on them too much, in the end they constitute a small group of all the situations and will come intuitively simply by mimicking Russians/ learning some constructed phrases by heart.