This is a pretty common question that I've answered before: https://www.reddit.com/r/AskHistorians/comments/9h2r91/why_were_the_armies_of_the_eastern_roman_empire/

Most of them were not mercenaries. Belisarius' secretary Procopius tells us that the 10,000 infantry were drawn from the foederati, barbarian vassals of Rome, and the professional comitatenses of the Roman field army. There were also 1200 of Belisarius' personal retainers, who probably were mercenaries, and 600 Huns and 400 Heruls, who were probably also mercenaries. Combined, only a small fraction of the army were mercs.