How do I copy a "purchase limit" field from a product to a product line item
You'll need to add a "copy limit" field to the line item first. If you just want to use the standard "product" line item that will work. Add a "copy limit" field at admin/commerce/config/line-items/product/fields

Then you need a rule that says:

Event: "After adding a product to the cart"

Condition: Entity has field: Entity:commerce_product, Field: field_copy_limit
Condition: Entity has field: Entity: commerce_line_item, Field: field_copy_limit

Action: Set a data value. Data: commerce-line-item:field-copy-limit, Value: commerce-product:field-copy-limit

You may then want to add the copy limit to your cart view.

However, If you really want to just validate that the amount in the cart is less than or equal to the copy limit, you may want to do something like this:


Set up as above. The display settings on your product display node add to cart field must be set to "combine like products"
Add a rule like this:


Events: 
* After adding a product to the cart
* After saving a new commerce line item
* After updating an existing commerce line item

Conditions:
* Entity has field: Parameter: Entity: [commerce-line-item], Field: commerce_product
* Entity has field: Parameter: Entity: [commerce-line-item..., Field: field_copy_limit
* Data comparison: Parameter: Data to compare: [commerce-line-item:quantity], Operator: is greater than, Data value: [commerce-line-item:commerce-product:field-copy-limit]

Actions:
*  Set a data value: Parameter: Data: [commerce-line-item:quantity], Value: [commerce-line-item:commerce-product:field-copy-limit]
* Show a message on the site: Parameter: Message: No more than [commerce-line-item:quantity] "[commerce-line-item:line-item-label]" may be ordered, so your order has been set to [commerce-line-item:quantity] items.

I put an export of this last rule at http://dl.dropbox.com/u/7350603/stackexchange/commerce_limit_items_in_cart.txt