Shaikh Al-Islām Abdul-‘Azīz Ibn Bāz (rahimahullāh) stated: Hasten with the fasting in accordance with what you think is correct. And Allāh does not burden a soul beyond its capacity. So if you estimate that you missed a certain number of days, then make them up. If you think it is [about] ten days, then fast ten days. If you think it is more than that or less than that, then fast in accordance to what you think is right. This is due to the saying of Allah: “Allah does not burden a soul beyond its capacity.” (Al-Baqarah: 286) And likewise, the saying of Allah: “Fear Allah and be dutiful to Him as much as you are able.” (At-Taghābun: 16)
(See: Majmū’ Fatāwā Ibn Bāz, 15/342.)
Note: This answer relates to the fasts that are missed for a valid Shariah reason such as menstruation, or a journey, or an illness, etc.