limit order quantity in drupal commerce
You can achieve this using Rules by following steps given below:


  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


Reference: https://drupalcommerce.org/questions/8066/how-limit-maximum-quantity-1-one-given-product-type