Loki simply doesn't have enough energy in his body to fuel this immense project, and he doesn't have energy stones to make up for the deficit in his body. He is dirt poor right now.
Fortunately, he has access to a seemingly inexhaustible source of energy that he can use as much as he wants. He is siphoning from that ocean of energy at a speed that is directly equal to his regeneration rate, which is a lot.
Another thing that these many tentacles require is a lot of mental power to control. After all, each tentacle is the equivalent of a rank 5 weapon.
As it is, he doesn't have that much mental power to control the tentacles right now. But he also doesn't plan to control the tentacles.
What he wants to do is to convert his armor into a large thorn plant that will fill up the room. As for the fact that he can't control this large thorn plant to make targeted attacks at specific individuals, he doesn't care much.
