The new adventures for Deathwatch are amazing, and they show how a small group of well trained and motivated Space marines can accomplish pretty much anything. In these 2 adventure, 4 Space Marines not only defend a planet against a tyrannid invasion, but also board the hive ship and manage to effectively shut down a hive fleet. If 4 marines can do that, imagine what more can do. As you add more the raw power multiplies so the true power of the legions astartes is revealed to the galaxy. No wonder they were able to conquer the galaxy back in the day. Even these are Deathwatch members, I am sure you could take 4 randomly selected Space Marines and get the same result. Space Marines are great not only for their stats, but also for their "can do" attitude.
5 Marines, a demi squad: An extra marine would probably be a welcome boost of firepower to make the adventures that much easier. Imagine 20% more firepower and how much of a difference that would make.
10 Marines: A full squad. Probably a ton of firepower which would make the adventures a cakewalk as the marines waltz through their encounters with tyrannids and rebels. Battles against hordes end instantly and every xenos they meet explodes in a shower of gore as ten marines unleash their bolters.
100 Marines: A full company. Probably the standard deployment of a Space Marine force. This same force of Salamanders held back an ork horde in Salamander. The same force of Red Scorpions in the Anphilion project was able to complete a mission agains another tyrannid horde. In the adventures prosented, The Marines take over the planet within hours, kill the rebels, and kill enough tyrannids to send Hive fleet Dagon screaming into space
200 Marines, 2 full companies. The UltraMarines and Mortifactors were able to hold back and defeat a hive fleet with such a force in the Ultramarines books. The Raptors were able to inflict heavy losses on the Tao in the Taros campaign (even if the campaign was a fiasco for imperial forces).. In the adventures, the rebels and tyrannids surrender as soon as the force spills out of the warp.
400 Marines: What survived on Rynns world after the missle accident. They stil manage to hold back an ork horde for over a year which numbered into the millions.
1000 Marines; A full chapter. According to the tyrannids book, the Ultramarines were able to destroy a whole hive fleet with their chapter (Kraken). They also didnt cheat and board the hive fleet to kill the hive queen. They actually fought and killed so many tyrannids that the whole fleet litterally burned through its biomass and ran out of soldiers.
Now, that is what a full chapter can do. Imagine that the lords of Terra felt inclined to put together a larger Space Marine force, imagine what they could do.
5.000 Space Marines, possibly the full Black Templars force. An unstoppable force, yet only about 0.5% of the full Legions Astartes. Could probably sweep into the Jerico reach and push back all three fronts in short order. Defeat and destroy a hive fleet. Wipe out a large Ork Empire. Exterminate some smaller races such as the Tao and Dark Eldar forever.
10,000 Space Marines, One hundredth of all Space Marines in the Galaxy. Bring the Jerico reach into line and take over a host of other sectors in the Eastern Fringe. Destroy a number of hive fleets which are still active. Send a message to any rebel systems that leaving the Imperium will be met instantly with force.
50,000 Space Marines. A full 5% of the total Space Marines everywhere. Probably something not seen since the days of the great crusade. Probably a large chunk of the total pre heresy legions not counting the Ultramarines. This is the stuff of epic battles which you read about in the Horus Heresy books. Able to destroy the Tyrannid threat permanently. Able to clean out most threats in the Eastern Fringe and make it a safe neighbourhood. Exterminate the Tao and spit out the bones as an after thought. Make sections of the Imperium ork free.
So, any other ideas of what a large Space Marine force can do?