I watched this years ago. I remember the main character was called Lock and he had to use these gems/ crystals to summon monsters to help him fight but usually you couldn't summon more than two as it was too much for the person to do.
You are thinking about Huntik: Secrets & Seekers, an animated series originally developed in Italy.
The series follows Lok Lambert as he uncovers the secret legacy of his missing father. His father, Eathon, was a member of the Huntik Foundation and a part of a secret world of magic users known as Seekers and creatures known as Titans. On his mission to uncover what happened to his father, Lok is joined by Dante Vale, Sophie Casterwill, Zhalia Moon, and a talking Titan called Cherit. Season one focuses on the fight against the evil Organization, led by a powerful Seeker known as the Professor [cut]
Titans are controlled by using some "amulets" that basically are gems/stones to which the monster is tied to. Since the summoner creates a bond with the summoned monster and "controls" it telepathically, it is pretty hard to control more than a few at once.