There are actually thousands upon thousands of Space Marines across the Imperium. As many thousands as there are chapters, with some exceptions to the Chapters to circumvent the usual rule of a 1000 marines per Chapter by being in a perpetual crusade against EVERYTHING and therefore allowed being over the limit (See Black Templars).
Space Marines remain of course, as others have mentioned, the most elite force of mankind while the men and women of the Imperial Guard (Astra Militarum now due to Games Workshop being a bit pissy about trademarking) form the humongous bulk of the Imperial forces with billions of troops across the galaxy. Often expected to loose millions of troops every day in the war against xenos and chaos.
That said, despite their expected expend-ability, the Guard are not without its heroes and noteworthy soldiers like Lord Solar Macharius, the only man who ever to lead a Crusade in the Imperium comparable to The Emperor:
Or Commissar Yarrick who ripped off the arm of a Ork warlord then using that same arm as a replacement for his lost one. He is considered legendary among Orks, which is damn mystical for a human to achieve, and is the only real worthy challenger according to the mightiest Ork to ever live Ghazghkull Mag Uruk Thraka.
Or Ciaphas Cain, Hero of the Imperium, swaggiest fucker in the galaxy, and awesome dude for completely different reasons than the ones above (read the books, they are great)
Despite this though, the Space Marines steal all the thunder and glory, especially Ultramarines, cause they are more often than not complete assholes and have their iconic power armor that is featured on the box of everything Warhammer.
Except the Salamanders, they are pretty nice dudes who likes sharing with the people. Primarch Vulkan being the nicest dude to ever walk the galaxy. Like, really nice.
No mention of Gaunt but a mention of Cain? I never like Cain's series compare to Gaunt's Ghost books, which are fantastic.