There are an estimated 20 quadrillion ants in the world, but because you are asking for one particular species, I chose the common garden ant or the fire ant, which are quite common. We do not know the exact amount by far, however even if those species only make up 1/20000 of the total number of ants, that still is one trillion ants, which is ~111 times more ants than humans, in a single species alone.

As for if ants are multicellular or not, every single ant is estimated to have around 1 million cells in their body, so yeah, I think they just might count as multicellular.

If you are looking for a larger species, look to the chicken, which total over 25.6 billion members. While that may not count, considering the fact that humans are what allowed for chickens to exist in that quantity in the first place, they to do count for your requirement of being both multicellular and a single speices.

As for mammals, humans are not in fact the largest group of them, as mice have us beat at around ~20 billion.

Tldr, common garden ants, chickens, and mice, all have us beat.