system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40k] If Space Marines were to land here on earth, how many could we take out?

...if any at all.
gpt
How many Astartes are there?  A squad?  A company?  A friggin' chapter?  What kind of support do they have?  What are they trying to do?  This is all necessary information to answer your question.

But if you want to know how Astartes would fair in combat with the armies of Earth...well, the Space Marines would slaughter Earth soldiers but would be overcome one by one.  Even small arms fire can eventually overcome a marine; and we have a lot more than small arms at our disposal.  A squad would be destroyed; a company might survive if it forced human military forces to join it.