I could go on and on about how much I dislike the work of Matt Ward, but when Roboute Guilliman divides the legions into chapters to prevent any one individual from holding so much power, yet those chapters all consider Marneus Calgar to be their "spiritual liege"...
pg. 48, The Zeist Campaign, 5E Space Marine Codex
...well, I just smell a rat is all. If Guilliman meant to prevent power from being held by one individual, yet all of these other chapters willingly submit themselves to Calgar, that means Guilliman failed in the long run.
But again, I could just be speaking out of dislike for Matt Ward. I wouldn't have a problem with the Ultramarines or their various ICs if he would just stop shoving them in my face like it's the best intellectual property to be written in the history of ever.
Ah but Marneus Calgar is shown to be more than willing to 'bend the knee' to Commander Dante, he recognises that whilst he is one of the greatest Chapter Masters currently alive he is still virtually a neophyte when compared to the Master of the Blood Angels- Calgar's flaw is that he's prideful but even he isn't that
Whilst Sicarius may have been placed in command of a combined force of Chapters he himself was more than willing to submit to the command of Logan Grimnar during the 13th Black Crusade, it's swings and roundabouts in the end.
Plus Calgar is renowned amongst the Ultramarines as being the greatest Chapter Master they've ever had, who is only eclipsed by Guilliman in terms of what he has achieved so considering the regular contact the Ultramarines strive for between themselves and other Astartes (a necessity considering how far on the edge of Imperial space they are) the fact that other Chapters are willing to heed the words of the Master of the most celebrated of Chapters is not too incomprehensible.
Only problem with using ten thousand as your base for that figure is that most of the legions, Ultramarines included, were larger pre-heresy. They might have started off at ten thousand each, but many of the legions grew in size and the Ultramarines were no exception to this.
The number of chapters they go on to create in the second founding would likely go on to support this as well. They have fourteen known descendants in the second founding, adding themselves that would mean that they were at least fifteen thousand strong.
9 known Chapters, including the Ultramarines, were founded from the Ultramarines Legion (Apocrypha of Davio) but according to the Apocrypha of Skaros 23 Primogenitor Chapters were founded in total (so that would 24 inc. the UM who are not a 'Primogenitor').
Legion sizes vary as we've all acknowledged and other than the 2 largest and the 2 smallest we have no idea the approximate numbers for each Legion really- recap:
Word Bearers- 100,000
Thousand Sons- 12,000
Emperor's Children 10,000?
Personally I think Guilliman was the greatest Primarch in his way, he made the difficult decisions that he knew might alienate him from his brothers but he did them anyway, everything he did was done to ensure the Imperium's survival- maybe there were underlying dreams of ambition, I don't know and couldn't really speculate, but in the end he succeeded with his intentions of leaving the Imperium a safer place.
He died an ignoble death but it was for mankind that he strived not personal glory or, necessarily, for the Emperor- that is the hallmark of true greatness.