If you missed these fasts without a valid reason (illness or traveling), then you must make them up ASAP. The days do not become double, you make up the amount of days you missed either before the upcoming Ramadan or after.

sheikh I wanted to know if you have a missing days of fasting and you haven’t make up those days and 2 Ramadans came after that how will you fast I already know that when the first Ramadan comes the the days become double