Excerpted from the B&C:
FAQ - How are New Chapters Founded?
This comes up periodically, and while GW has been unclear on the subject, this particular theory has gained acceptance on Liber as logically being a good explaination. The first three points are GW canon, and inviolate, with the rest being the favoured way to explain the known facts. The High Lords may have done things differently over the last ten thousand years, but this should give a template to work from.
1. The High Lords of Terra, speaking for the Emperor decree a new Founding of Adeptus Astartes Chapters. There is great jubilation and much quaffing.
2. The Adeptus Mechanicus on Mars selects geneseed from the tithed stocks, say from the Ultramarines / Roboute Guilliman gene-line, although as there are only nine First Founding Chapters out of the thousand possible, it is most likely that the geneseed selected will actually be from a successor Chapter.
3. The Adeptus Mechanicus cultures some more geneseed from the original sample selected using mindwiped subjects floating in big glassteel tanks. They would also supply some equipment (Rhinos, ships, armour) to get them started, with the rest expected to be produced by the new Chapter's Forges, or requisitioned like every other Chapter does.
4. A small cadre of experienced marines from the chapter that supplied the geneseed is given the honour / responsibility of training the initiates from the fledgeling chapter. This would ensure that they follow (broadly) in the footsteps of their parent chapter from a doctrinal and philosophical point of view, although there is still the potential for deviations... children never turn out exactly like their parents.
5. The cadre could be made up of a captain from the parent chapter to be the first Chapter Master, along with experienced marines to become the training scout sergeants, and eventually marine squad sergeants and captains. There would also be specialists like a Chaplain, Techmarines, Apothecaries and Librarians to get the chapter going and train up a new generation from among the initiates.
6. This would number between 20-40 marines, and there is no way that the First Founding chapters could do this for multiple (up to 20 at a time) successors. Again this supports the idea that later Founding Chapters would have to have a hand in founding the new Chapters, so while two thirds of Chapters are from the Guilliman geneline, the actual geneseed and initial cadre of Marines might have come from the Mortifactors, Black Consuls, Eagle Warriors or any other Chapter with Guilliman geneseed as well as directly from the Ultramarines themselves. For ease of explaination GW (and the Administratum) may call all of them with the Guilliman geneline 'Ultramarines successors', and there is always the chance of snobbery existing, with a chapter wanting to call itself an Ultramarines successor when technically it is the successor of a 25th Founding Guilliman Chapter that no-one has ever heard of.
7. The Chapter will recruit and grow in numbers until after many decades it reaches the full compliment. It will participate in battles and campaigns as it's numbers allow, perhaps fighting alongside other Chapters until it has sufficient numbers to conduct independant actions. Remember that a Chapter will rarely send its whole compliment of ten companies to war other than in the most dire of circumstances, so squad and company level actions are common and will be possible quite early in the Chapter's history.
8. The initial cadre of marines may return to their parent chapter once they have got the new Chapter running - they may always be Crimson Fists at heart. Alternatively they may elect to stay. Going back to being a Captain after having been a Chapter Master might not appeal to many. It could depend if the secondment was temporary, or of a more permanent nature.