Who is Pushyamitra Sunga?

I presume that the OP is referring to the “Pushyamitra” who appear in the context of the Campaigns of Emperor Skandadeva.

We don’t know.

There are no references to this tribe or community or kingdom or whatever they might have been - except for a single mention in one of Emperor Skanda’s proclamations (save one dubious reference in the Puranas as mentioned in the Wiki as well). This is especially surprising for two reasons:-

1. Imperial Inscriptions of this sort were meant to mark momentous happenings and the Emperor would hardly refer to some nameless band of malcontents in the same vein as the Hunas- who’d been clashing with the Empire for the better part of two centuries, in addition to claiming continuity from the line of the Kushana themselves.
2. The existence of a Military Power capable of challenging the Gupta Field Army under Emperor Skanda- itself one of the mightiest armies raised in the history of the Continent, let alone that of the Gupta Empire- is simply not possible.

A more likely option is that the “misreading” theory - again, referred to the Wiki link provided- is the more plausible of the two options; the change of a single letter in the Translation gives the more prosaic “Enemy War-Coalition”- most likely referring here to the Vakataka-Mekala Alliance that briefly took over and occupied the city of Mathura during the height of the Gupta Civil War simultaneously as the Hunnic Advance was at its apex of Power.

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Comment: How mighty was his (Skandeva’s) army ?

Cat: 200,000 is the figure in the Shastras.