The cover page says "a2 Tenore e Basso".
At the beginning of the Gloria, there is an indication "Alto soli". However, the top stave stays in tenor clef, so the "Alto" could possibly refer to violas playing along with the vocal line.
Throughout the score there are various tutti and solo/soli indications.
Grove Music says "Those works conceived for the whole cappella are intended to be performed with the four vocal parts taken by one or two singers (the second singer called ‘di rinforzo’)". Accordingly, this is tagged for tenor/bass solos with tenor/bass choir.
Grove goes on to say "usually accompanied by orchestra (responsories and Lamentations are accompanied only by violins and violas). Works for ordinary circumstances have only a figured bass accompaniment, performed on the organ." Tagged for organ continuo, and if a fuller score surfaces at some point, an orchestral tag can be added.
