After being resurrected, Roboute Guilliman, Primarch of the Ultramarines Chapter, wrote the Codex Astartes, the Holy Tome of the Adeptus Astartes. It's essentially a giant rulebook for how the Space Marine hierarchy is structured. Amongst the many, many changes he made, Guilliman limited the Adeptus Astartes Legions (some of which having had millions of Battle-Brothers within them) to 1000-Astartes Chapters. The remaining Space Marines unaccounted for by the downsize were formed into Successor Chapters, sharing gene-seed and traditions with the Parent Chapters from whence they were born. However, the Ultramarines still remains one of the largest Chapters, boasting more (known) Successor Chapters than any other Parent Chapter (those being the original Loyalist Legions created by the Emperor from the line of the Primarchs). Some of the people I've talked to even claim the Ultramarines have far exceeded the 1000-Astartes limit, hence why they're all over the place in the books and games.
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