Must one make up for six days of fasting from past Ramadans, due to the assumption of dryness and not knowing the sign of purity from menstruation?

If the bleeding stopped on the fifth day with traces remaining, then waiting until the sixth day is correct and you do not owe any missed prayers. However, if the cessation means complete dryness on the fifth day, and you delayed performing ghusl until the sixth day, then this is an error, and you must make up the prayers you missed after becoming pure, estimating their number based on what you believe most likely absolves your responsibility.