Learn Hardcore Latin: Decem testes idem dicunt: servum innocentem esse et nihil cepisse. - Ten witnesses say the same thing: that the servant is innocent and took nothing.

Because testes is nominative plural, and it matches the plural verb dicunt. Its dictionary form is testis, meaning witness. So decem testes dicunt means ten witnesses say.

Why does decem not change its ending?

Because decem is an indeclinable numeral. In Latin, ten keeps the same form no matter what noun it goes with. So you get decem testes, decem servi, decem feminae, and so on.

What exactly does idem mean here?

Here idem means the same thing. It is the object of dicunt.

A learner might expect something like iidem, but that would mean the same people/men as a nominative plural. Here the sense is not the same witnesses say..., but the witnesses say the same thing. So idem is neuter singular: the same thing.

Why is there a colon after dicunt?

The colon introduces the content of what the witnesses are saying. In other words, everything after the colon is the statement being reported.

It is also worth knowing that punctuation in printed Latin is editorial. Ancient Latin was not originally written with modern punctuation, so the colon is there to help the reader see the structure.