The Shape&Material bonus to your lab total is calculated per device, but you certainly don't get to add together them if you tried to create multiple items of the same sort.

(Assuming I understood what you are after)

So if your lab total without any Shape&Material bonus is 18, and you got a S&M bonus of +2, then you have a lab total of 20.

If you want to create an invested item you first need to open the item for enchantment. This process does not use your lab total in any way, but just involves spending an amount of vis depending on the size and material(s) of the item. I see no indication in the rules that you can open more than one item per season.

Opening the item is not needed for lesser enchanted items.

Then you need to invest effect into the item. To be able to do this in a single season you need to have a lab total twice the level of the invested effect. So if you wanted to invest a level 1 effect in one season, you'd need a minimum lab total of 2 - including any Shape&Material bonuses.

This is true for both invested items, and lesser enchanted items. (Lesser enchanted items can only be created if you can do them in a single season)

Now, using the rules for multiple labratory activities you could invest powers in several identical items in a single season. For our hypothetical item you could create 10 of them in a single season. ( lab total of 20 divided by the required lab total of 2)

If you want to create 20 invested level 3 items in a single season, you need 20 opened items lying around, you need a lab total of 6 to invest each of them in a single season, so you need a Lab Total of 20*6=120 to pull it off. No, you do not get to count the Shape&Material bonus 20 times for the Lab Total any more than you get to count your Magic Theory bonus 20 times.

You'd also need to spend 20 pawns of Vis and thus need a Magic Theory score of 10 or higher.

One question that pops up is if you try to invest effects in multiple different items (with different Shape&Material bonuses) in a single season, but all using the same Technique+Form combination. This seems to be allowed according to RAW. What is not clear is how you'd calculate the lab total - since it could be different for each item.

My ruling would be that in this case you'd use the lowest applicable Lab Total to calculate how many items you can invest powers in during that season.