What is the ruling on money that a person took from his friends as help and a debt, of which he has forgotten some and remembered some, and is he obliged to repay it, or should he ask each of them about their debt?

You must repay all debts that are legally proven, according to the Almighty's saying: (Then let him who has been entrusted fulfill his trust and fear Allah, his Lord). As for what was not mentioned or not legally proven, you are not obligated to return it. It is more prudent to satisfy anyone who claims a right against you and to seek their forgiveness. However, for what was taken as assistance without a condition of compensation, you owe nothing for it.