There are more then 1 million Astartes in the galaxy. The million number is quoted based on the idea that 1000 chapters, and the codex organisation calls for 1000 marines. But, this does not take into account the specialist roles (medics, librarians, command staff, tech marines, tank and other vehicle crew, if the chapter has its own fleet there will be marines commanding those vessels, and the astartes pilots commanding the fighter ships) it also doesn't take into account the fact that most chapters run with a surplus to be able to handle sudden large losses in war. Considering it takes some chapters years to find and train aspirants there is always a ready stock of marines waiting and ready to replace fallen comrades, plus given that some chapters are spread in various sectors sometimes for years it is highly likely that recruits will be absorbed into company ranks without paying attention to how many recruits other companies have trained.
It also doesn't take into account the number of chapters that do not follow the Codex, the Space Wolves have 13 great companies, each made up of more then 100 marines, the Black Templars is thought to number 6000 marines and there will be additional chapters that break the rules.
In terms of the other resources.
Titan legions - A titan legion consists of roughly 50 engines, mainly Warhound and Reaver with Warlords more rare and Imperetors now rarer still. From reading various sources during the heresy each Forge World had at least 1 titan legion, with the larger more powerful forge worlds possibly having a legio per forge (mars for instance had a number of titan legions on it) if you work off roughly 1000 forge worlds in the galaxy capable of maintaining a Titan Legion and work on an average of 2 titan legions per forge world that gives a rough number of 100,000 Titans in the galaxy during the Heresy. Post heresy the number is probably less, although Gamesworkshop has changed the fluff around how easily new titans can be produced (going from very few forge worlds being able to, to now suggesting more forge worlds have the ability at least to build the smaller titans)
Fleets - It is harder to guess the number of warships the Imperium has, lets first consider the numbers above, 1000 chapters, all of which have some form of space fleet, lets be conservative and say 25-75 Battleships of varying sizes, some chapters may have far more while others are limited to just 2-3 vessels. based on 30 battleships per chapter that's 30,000. Then we have the fleets attached to each Titan Legion, going from the stories I have read when a Titan legion goes to war it has not only the Titan Transports, battleships in there own right with armanants capable of defending the vessles, it also has a flotila of Adeptus Mechanicus warships supporting it, lets say on estimate, a 50 strong titan legion has 15-20 Titan Transports depending on the size and make up of the titans, and then a further 50 support battleships. that's 100,000 titan legion vessles, and that is a conservative estimate
Then you have the Imperium Navy, Battle Fleet Solar on its own is large enough to defend the Solar System while also engaging in campaigns many systems away from Earth (most recently it supported Cadia during the last Chaos incursion) so we can assume it is made up of many many thousands of warships. Then consider every populated system has its own fleets, and there are multiple other fleets attached to the imperial navy you are probably at over 1,000,000 battleships in total throughout the galaxy.
The Imperial Guard then would be a huge number, Consider the Gaunt books, they engage in battles that are part of wars involving 10's of millions of guardsmen across whole systems, this is one war in one tiny part of the galaxy. So you are probably looking at billions on billions of Guardsmen across the galaxy, and that number is constantly changing as new armies are formed and wiped out.
However if you want to go with the official answer, several authors have stated that there are as many of a thing as the story dictates, that goes for the Eldar, Tyranids, Tau and the armies of the Imperium.
