Is there a limit for how many cards you can have in your hands or not? i mean can you have only 6 or can you have 20?

You can have as many cards in your hand UNTIL the end of your turn, by the End Phase of your turn you must only have a maximum of 6 cards left in your hand. Unless there is an effect that says otherwise like Infite Cards.

The only limitation on hand size is...

In the end phase of your turn you may only hold a maximum of 6 cards in your hand; if you hold more than 6 cards, then you must discard cards until you have 6.

At any other point in the game, you may hold any number of cards.

Examples of "changing" the limit...

Infinite Cards
Hieroglyph Lithograph
Enervating Mist