How to limit the products from the compare in drupal?
You may solve this through Rules module.

Event: After udpdating an already existing line item

Conditions:
1/ data comparison: commerce-line-item:type = product
2/ data comparison: commerce-line-item:commerce-product:type = Your given product type
3/ data comparison: commerce-line-item:quantity is greater than 1

Action:
- set data value: commerce-line-item:quantity 1
- display custom message