“I find it hard to believe that you're really going to need all that luggage for just 3 days.”

Your daughter is packing for a weekend vacation. She's packed two large suitcases and a backpack.You think that's too much. You say:

I find it hard to believe that you're really going to need all that luggage for just 3 days.

I find it hard to believe that (clause)

"I find it hard to believe..." actually means that you don't believe something. The first part, "I find it ___", means "I think it is ___":

I find it hard to relate to him sometimes.

I find it really hard to focus on my work in the afternoon.

Use "I find it hard to believe..." to complain about something that you don't think is true. In the example above, the speaker doesn't think that her daughter really needs so much luggage. So she says "I find it hard to believe that you need all that luggage".

Grammatically, you should follow "I find it hard to believe" with a clause connected by "that". The clause is like a little sentence:

I find it hard to believe that you wrote this paper yourself without any help whatsoever.

luggage

"Luggage" is stuff like suitcases, bags, and cases that you take with you when you travel somewhere.

The word "luggage" is uncountable, so people don't normally say "luggages." Other related words, like "suitcase", are countable:

I find it hard to believe that you're really going to need that many suitcases for just 3 days.