The telomerase is an enzyme that adds telomeres.
What stops the telomerase from adding too many telomere sequences ? What's the regulating mechanism ?
Telomerases are tightly controlled on all level: Expression, post-translational modifications and by external factors. I think for this the external factors, a protein class called telomeric proteins. They bind to the end of the telomere and regulate the telomerase.
The figure is from this paper, which looks into the topic quite extensively: