If you want to harmonize the accounts, probably there were two angels.
Were they men or were they angels?
Both Mark and Luke say they were men dressed in white robes, which can easily be understood to be visions of angels. Especially in Luke's account this is obvious, since it would be unusual to describe a man with a robe that "gleamed like lightning." In fact, while Luke initially reports in 24:3 that they are two men with gleaming robes, by 24:23 the disciples on the road to Emmaus explain that the women had seen a vision of angels. Perhaps Mark and Luke report their appearance as men, since at first the women did not seem to understand what they had seen and only later realized they were angels.
How many angels were there?
The fact that Mark and Matthew only reports a single angel doesn't preclude there from being another present. Neither insists that there was only a single angel present; they simply record that there was an angel present who spoke to the women to reassure them and to give an explanation for why the tomb was open and empty. Most scholars today think that Luke had access to Mark's account, so the fact that Luke goes his own way suggests that he saw no contradiction in giving a bit more detail.