Currently there is 60 heroes in Dota Underlords. Valve did not introduce new races and classes in Dota Underlords, which are called "Alliances" here.
Already in the early version of Underlords a couple of new characters are available. For example, Bloodseeker - joined the ranks of the murderers, and Arc Warden became a new elemental. Pudge lovers also did not hurt, and even left him a Meat Hook.