Exegesis:

anepesan prasiai prasiai ‘they reclined in ranks.’

anapiptō (8.6) ‘lie down,’ ‘recline’ to eat.

prasia (only here in the N.T.) meant originally ‘a garden plot’; when used as here it means ‘in orderly groups,’ ‘in rows,’ ‘in ranks’ (cf. Moulton & Milligan). The element of order is stressed in the use of the word: the multitude formed orderly rows which could be easily and quickly served by the disciples.

kata hekaton kai kata pentēkonta ‘by the hundreds and by the fifties’: so most translations and commentaries. Manson, however, has ‘a hundred rows of fifty each’ (“a great rectangle, a hundred by fifty …: ‘one side of the rectangle was reckoned at a hundred, and the other at fifty.’”): this, however, has not commended itself to many (cf. Lagrange “bien mathématique!”).

Translation:

In groups, by hundreds and by fifties is a very compact phrase, and one which must in certain languages be somewhat expanded, e.g. ‘different groups; some groups had one hundred people and other groups had fifty people’ (Southern Subanen); Toraja-Sa’dan expresses it ‘in groups there were hundreds, there were fifties.’

Quoted with permission from Bratcher, Robert G. and Nida, Eugene A. A Handbook on the Gospel of Mark. (UBS Handbook Series). New York: UBS, 1961. For this and other handbooks for translators see here .