NOTATIO

Usually on the top of the first preface pages we have "instrumentation", but in the case it is mixed ensemble with voices, is the word "instrumentation" also applicable?
Also, if we have various vocal groups with soloists, but without any instrument, do we still call it "instrumentation"?

As far as I know, the voice is "instrument" as well, but I am unsure about the second example.

FWIW I usually list voices and instruments separately.

There are various generic terms or phrases which might be used to cover both voices and instruments. At its simplest perhaps (eg): ‘For Tenor Solo, SATB chorus, 2 Violins, Viola, Cello and Double Bass’ or ‘To be performed by Mezzo-Soprano and Baritone with Flute, Clarinet and Viola’ etc etc.

In French we have the word "effectif" that can be used in this type of case. (effectif: the number of real persons constituting a group)

Is there possibly an English equivalent?

"Forces"?
Sounds terrible...

I don't think there is any single preferred way to do this. However, the title, then if appropriate a description (eg cantata, sonata etc etc etc), then ‘for’ followed by a list of voices and/or instruments would seem simple and clear.

Perhaps a list of the instruments under the title "Orchestra", or "Instruments of the Orchestra" and then the vocal forces listed separately if complex, or not at all if they are listed as part of the title or are characters in an opera? That's what I am seeing having looked through several late 19th and 20 century scores. On the other hand, Stravinsky lists "chorus" centered after the list of instruments all under the heading "Orchestra" in his Symphony of Psalms.

As David says there seems to be no general agreement on this interesting question, but using the term "instrumentation" does seems strange to me for voices, unless the voices are actually used as instruments in the orchestra, perhaps wordlessly as in Daphnis or The Nutcracker.