Could you show me some very easy mate in 20? I am looking for some puzzles where there is a forced mate in as many moves as possible (ideally 20 or more). Yet the puzzles need to be as easy as possible!

You will probably need to find some puzzles where the defending side has a lot of spite checks to delay the mate.

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Edit :

I just made a very easy mate in approximately 23. White to move:



It's really really easy since Black can do nothing. But it doesn't totally satisfies me since the position is of course not legal...
There are many:

     [Title "Mate in 20"]
 [FEN "3K1B2/1p6/pp6/rk2N3/b1p5/1pP5/1P2P3/8 w - - 0 1"]


You should search puzzles from the 1900. It was a golden era for chess puzzles. I found a mate in 63 some days ago. 

Chessmaster also has a nice collection of interesting(and long) puzzles. There are many sites where you can sort puzzles according to difficulty and turns to mate, but I can't remember any atm.