I personally would recommend having and preparing two backup doses of the same size as the primary dose. There is also a benefit in having some spare for testing purposes (as in with test strips, not by taking it). Also, there is always a chance you will prepare the mixture and then be interrupted and have to postpone your attempt, or knock it over accidentally, or change your mind on the day. Also, if you can afford to buy a double quantity, then you can store half of it somewhere separate to reduce the chances of discovery or seizure. Having some extra is a good insurance policy in those scenarios, especially as SN may become more difficult to obtain in the future. SN should be quite cheap from most sources, and even if the shipping is particularly expensive in your case, I would be surprised if it increased much based on relatively small changes in the amount ordered.