Kanji in Japanese language writing

Hi everyone
I am learning Japanese and I feel a bit confused of Kanji writing. I have a question that " Is that alright if I use only Hiragana and Katakana to write?" cuz for me, it's not easy to master the Kanji characters even though I had some Chinese Mandarin course before. I found it really difficult to remember the Chinese characters and now I encounter the difficulty again when seeing Kanji in Japanese lessons :(

In informal setting? Yes.

How many Kanjis do we need to know to master?

I have no idea xD ..yes I think so..that's ok to use only hiragana in informal talk :d