I've played for quite some time now, and I've found only two red mutagens so far, both are named ones and not the generic red mutagens. I've at least a dozen of the green and blue mutagens lying around.
Where can I get some more red mutagens from?
Find red mutagens
the-witcher-3
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Best Answer
According to this wiki, they drop from the following monsters:
Archgriffin, Doppler, Katakan, Greater Foglet, Nekker Warrior, Water Hag, Wyvern, Succubus, and Werewolf
As you can see, they aren't the most common monsters, which is likely why you don't have as many as blue or green. This link has a couple farming locations.