In fact, there is no clear evidence on this matter, unless someone were to say: Our evidence is the hadith of Ka‘b ibn ‘Ujrah, when the Prophet ﷺ permitted him to shave his head and to feed six poor persons, giving each poor person half a sāʿ (1). Thus, he specified half a sāʿ for each poor person, and the rest may be analogized to it. The matter is only approximate; it is not a fixed, established limit.
If you reflect, you will find that charitable feeding—and those fed—fall into three categories: at times, what is given is specified while the recipient is not; at times, the recipient is specified while what is given is not; and at times, both what is given and the recipient are specified.
An example in which what is given is specified but the recipient is not is the zakat al-fitr: it is fixed at one sāʿ for each person, but those to whom it may be given are not specified. Therefore, the fitrah charity may be distributed among more than one poor person, and several fitrah payments may be given to a single poor person. Thus, in this case, what is given is specified but the recipient is not. Or, if you prefer, you may say: what is paid is specified, but the one to whom it is paid is not.
An example in which both what is paid and the recipient are specified is the expiation for harm. The Messenger of Allah ﷺ said: “Feed six poor persons, giving each poor person half a sāʿ.” (1)
An example in which the recipient is specified but what is given is not is the expiation for an oath. For this reason, Shaykh al-Islam said: So long as the Sacred Law has not specified