We do if we want to preserve the little bit of wild are we have left and keep a decent quality of life where we can eat fruit and vegetables. Even cutting meat to zero isn't going to be enough.

It's an unpopular opinion for sure. But as it stands, our planet cannot sustain the way we exploit it. Our seas are being emptied and poisoned, we cut rainforest to be used for overcropping. It's what happens if you have 8 Billion people on a relative small planet. Sure we could do better but that's not in our nature untill doom stands on our doorstep.

It's actually quite popular among environmentalists. We need to make progress on sustainability, and we will. We CAN sustain 8 billion people because of the immense progress humans have made in food production and efficiency. Pretending the only solution is, less humans, is antithetical to everything humanity has accomplished.

Sorry for the pedantry, but Earth is not a "relatively small" planet. It's actually quite large for a rocky planet. Out of all the rocky planets and moons in our own system, Earth is by far the largest (except for Venus, which is about 90% the size). From what we're seeing with exoplanet research, it looks like Earth is still pretty large, though it's hard to tell since we have a hard time detecting smaller planets, and the "super-Earths" are more easily visible to us.

Of courses, compared to gas giants, Earth is small, but that's like comparing apples and whales.

Why do we need billions of humans?

"we" is doing a lot of heavy lifting in your sentence but we're about 2 centuries away from complete population collapse of the species.

There are less babies born on this day than any day in history, and that will be true for the rest of our entire species' existence, barring major technological "Brave New World" style test tube baby advances (which are indeed extremely likely given this fact.)

Only if people somehow will learn how to spend less of everything.

If you don't produce more people, someone else will.