किमः संख्यापरिमाणे डति च

The affixes डति (अति with the elision of इम्) and वतुप् come in the sense of 'numerical quantity' after the word किम् in the Nominative 1st-Case in construction.
The च introduces the affix वतुप् which is changed into घतुप् (इयत्) ॥

The word संख्यापरिमाणे means संख्यायाः परिमाणं i.e. संख्यापरिच्छेदः 'making estimate by numbers'. As का संख्या परिमाणमेषां ब्राह्मणानां = कति ब्राह्मणाः or कियन्तो ब्राह्मणाः 'how many Brahmanas in number do you estimate these'. This always comes in the plural.

This word is formed, in fact, when a question is put relating to the numerical quantity (sankhya-parimana) of any thing. But the word sankhya-parimana may be explained also as a Karmadharaya, in which the attributive word has been placed second. It will then mean "quantitative number". The sutra will be rendered thus: "kim takes dati, when it means a quantitative number". It should not be objected that numbers are always quantitative, and therefore parimana is redundant in the aphorism. For sometimes numbers are used not to define any quantity but merely to mark contempt. As in the following:-

अव्रतानाममन्त्राणां जातिमात्रोपजीविनाम् । सहस्रशः समतानां पारषत्वं न विद्यते॥ Here the word सहस्रशः is used indefinitely in a contemptuous sense. Therefore, it is not true that a number always defines a quantity. Where किम् is used to express contemptuous number, no affixing takes place : as केयमेषां संख्या दशानाम् ॥ The word parimana in this sutra is not used in its technical sense; as in Sutra 5.2.39 ante: for its very repetition here shows that it is used here to denote 'quantity' in general, and not 'capacity', for a number cannot mean 'capacity'.