Grab your calculators. Let's talk some Astartes numbers.
The Primaris Space Marines are the largest single addition to the Adeptus Astartes, and the most reinforcement the Imperium has received since the end of the Horus Heresy. These enhanced warriors are an improvement upon even the Emperor's own work, His Angels of Death. Not only are they individually mighty, and armed with new and powerful weapons, but they come in staggering numbers. Across the wide galaxy as part of the Indomtis Crusade hordes of the newly awoken Marines have worked to reestablish Imperial control. But just how many Primaris Marines are there? Let's take a look.
Sadly we aren't given any solid total numbers here, but we do have some things to work with. When Cawl delivered the Primaris to Guilliman, he divided them into two groups. Half of the new Primaris were formed into brand new Chapters of their own. The other half were formed into a group known as the Unnumbered Sons or the Greyshields. The Greyshields traveled with the Indomitus Crusade and fought as their own formations. Besides, they were used to form new Chapters as needed and to reinforce existing chapters along the way.
At what was the end of the Indomitus Crusade at the Battle of the Pit of Ruakos, now just the end of the first phase, there were still 20,000 Unnumbered Sons, as stated ...