The Reapers would harvest humanity (kill them) at a rate of several million per day a number that would increase as they used their husks to construct more Processor Reapers, i'll leave the math to you
And we don't know if Shepard took weeks or months...Scary thought if it were real life.
Millions. Billions, possibly. As has been said, the Reapers were harvesting/killing over one million people per day, and that's not counting the times they just decided to straight up decimate population centers without bothering to harvest (e.g., when they deployed an orbital strike on Adelaide). Shepard mentions in conversation with Tali that it took several months to rally the galaxy together before returning to Earth, which means that the Reaper invasion was almost certainly the most devastating genocide in human history by far.
Quoth the Codex: "The rate of killing is phenomenal. Intelligence estimates suggest there are more than 400 processor ships on Earth, killing approximately 1.86 million humans per day. In combination with battlefield deaths, disease, and famine, this pace will result in the complete depopulation of Earth within a decade. As the husks and indoctrinated slaves build more slaughtering facilities, the kill rate can only increase."
If the events of ME3 took place over the course of a year (which is the minimum amount of time I think it would take), that's about 680 million people dead as a direct result of Reaper processing. This does not include deaths from the inital assault, Anderson's resistance, or the final Hammer battle, which are all likely to be high. Luper567 (talk)
Processor ships killed 1.8 million humans per day on Earth and war lasted 2 months top. That makes about 60 million humans killed by processor ships plus other casualties that would make somewhere around 65-70 million cassualties on earth plus somwhere around 10-15 million on other planets and in space battles.
If figures on Earth were anything like Palaven, with three million and five million dead on the first and second days of fighting, respectively, then, even averaging it out to four million dead per day, that's 240 million dead in two months for Earth alone. Considering that most professional military forces were forced into reatreat or wiped out within hours of the Reaper assault, ground combat would have primarily taken place using vast numbers of rapidly trained conscripts and volunteers. As the professional military units were wiped out, casualties among these poorly trained recruits would increase. The assault forces for Hammer and Sword must have numbered in the thousands of warships and millions of soldiers (assuming all factions were acquired), tacking together whatever was left of the Turian, Asari, Salarian, and Human fleets, plus the Quarian Heavy/Patrol fleets, at least two Liveships and a chunk of the Civilian Fleet, the entire Geth fleet, plus Hanar, Drell, Batarian, Elcor, and Terminus forces, and that's complemented by Krogan ground troops. Hammer is completely combat ineffective by the end of the fighting in London, with around 50% of the entire group shot down just on approach to the city. If the casualty rates in space were anything like those on the ground, the Quarians alone (assuming the entire Migrant Fleet joined the fighting on Earth) would have lost at least eight and a half million people and 25,000 ships.