2

according to this link http://www.germanveryeasy.com/adjective-declension

In the weak declension category, it says if the adjective is indicator of quantity like mancher then it's weak declension while manch is in strong category

With both have the same meaning "some" then what is the difference.

3

Read carefully. The site says that weak declension is used for declined indicators of quantity, an example being mancher. Manch is just not declined.

This is how declination of manch works (from de.witkionary.org):

           |            Singular            |  Plural
           | Maskulinum Femininum  Neutrum  |    ——
-----------|--------------------------------|------------
Nominativ  | mancher    manche     manches  |  manche
-----------|--------------------------------|------------
           | manches               manches  |
Genitiv    | manchen    mancher    manchen  |  mancher
-----------|--------------------------------|------------
Dativ      | manchem    mancher    manchem  |  manchen
-----------|--------------------------------|------------
Akkusativ  | manchen    manche     manches  |  manche    

A more in-depth review of manch can also be found at Canoo.

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