G-d sent thousands of prophets in Bnei Yisrael but the Bible mentiones only 55 prophets. Do we believe in those prophets who are not mentioned or reported in Bible? Are they recognized as prophets?

All true prophets are true and all false prophets are false. We believe in all true prophets. The true prophet gets his prophecy from Hashem and a false one imagines things or makes them up. The Tanach only records prophecies of 47 prophets and 8 prophetesses since they were the only ones that prophesized prophecy relevant to future generations. Since the many hundreds of thousands of the others are not recorded in the Tanach we don’t know their names nor their prophecy. Prophecy ended with the last prophets of Tanach (Chagai, Zecharya and Malachi) and will return before the redemption of Mashiach.