What is the wisdom behind mentioning the number "twenty" and "one hundred" in verse (65) of Surat Al-Anfal, then alleviating it by mentioning "one hundred" and "one thousand" in the subsequent verse? And does this mean that fighting is not obligatory for believers if their number is less than one hundred after the alleviation, or less than twenty before the alleviation?

The text of the two verses is: "O Prophet, urge the believers to fight. If there are twenty among you who are patient, they will overcome two hundred. And if there are one hundred among you, they will overcome a thousand of those who disbelieved because they are a people who do not understand. Now Allah has lightened [the burden] for you, and He knows that there is weakness in you. So if there are one hundred among you who are patient, they will overcome two hundred, and if there are a thousand among you, they will overcome two thousand, by permission of Allah. And Allah is with the patient."

The meaning before abrogation was that it was not permissible for one Muslim to flee before ten disbelievers, nor for ten before a hundred. After abrogation, it is not permissible for one to flee before two, nor for ten before twenty.

Al-Shawkani said: "They were commanded that a group of them should stand firm against ten times their number. Then, when that became difficult, Allah lightened the burden for them, making it obligatory for one person to stand firm against two disbelievers."

And in Tafsir Ibn Kathir, it is stated: "Then this command was abrogated, and the glad tidings remained, and Allah lightened the burden regarding the number, and reduced the [required] patience, to the extent that He lightened it for them."

Al-Bukhari narrated from Ibn Abbas regarding this verse: "It was prescribed for them that twenty should not flee from two hundred. Then Allah lightened the burden for them, saying: 'Now Allah has lightened [the burden] for you, and He knows that there is weakness in you.' So it is not appropriate for one hundred to flee from two hundred."

Ibn Abbas said: "When this verse was revealed, it was difficult for the Muslims, and they found it immense that twenty should fight two hundred, and one hundred a thousand. So Allah lightened the burden for them and abrogated it with the other verse. So, if they were half the number of their enemy, it was not permissible for them to flee from their enemy. And if they were less than that, it was not obligatory for them to fight them, and it was permissible for them to withdraw from them."