Peace be upon you. If you are unsure about the number of missed prayers and are uncertain whether it is fewer or more, you may suffice with the amount you are certain of, and you are not required to make up the amount you are unsure about.
Source: Ayatollah Sistani, Selected Issues, Issue 440.
Additionally, Ayatollah Khamenei states that if someone has several missed prayers but does not know how many, such as whether they missed two or three, it is sufficient for them to make up the lesser amount (the amount they are certain they missed).
Source: Educational Message, Lesson 54: Daily Prayers (20), Missed Prayers - Rental Prayer - Making Up Parents' Prayers.
May you be successful in all good.