Suetonius compares the belligerents Caesar and Pompey, in regard to their treatment of Romans who
refused to take either Pompey’s or Caesar’s side (Suet. Jul. 75)
I’m making a lot of grammar calls of which I’m unsure, but I seem to remember more when I try, and then see the correction.
denuntiante Pompeio pro hostibus se habiturum qui rei publicae defuissent, ipse medios et neutrius partis suorum sibi numero futuros pronuntiauit.
denuntiante Pompeio: ablative absolute [with Pompey declaring]
pro hostibus se habiturum: indirect discourse, main clause [that he meant to treat as enemies]
qui rei publicae defuissent: indirect discourse, subordinate clause [ the ones who had failed the public good, i.e., had not been Pompey’s active friends ]
The preceding is subordinated in its sense to the clause declaring Caesar’s atitude.
ipse: nominative singular, he himself, i.e. Caesar.
medios et neutrius partis: the in-betweens, and those of neither party. medios seems to be an adjective, accusative plural, while neutris partis is genitive singular; maybe both complement a reader-supplied illos.
suorum sibi numero futuros pronuntiauit: This was the hardest part. In general I believe it means something like, “He declared that those not against him would be accounted as friends to become.”
suorum: of his own side. I judge this the genitive complement of numero–see below.
sibi: I want to call this dative of reference, Allen and Greenough, no.378, the Dative of the Person Judging. In English I think we might say, “He declared that in his judgment those not opposing him were friends in the making.”
numero: the ablative singular of numerus -a -um, an adjective. I think it has noun force here, meaning “in the category”.
futuros: I suggest this is an incomplete future active infinitive, with futuros esse filled out by the reader.