Not necessarily at once. I would prefer to use them in case my app gain popularity in the future.
I have also noticed an odd thing: when I checked code expiration date today it showed July 8th, 2020 (few days ago it showed July 4th), so it seems that expiration day changes and always shows a date in one year.
One promotional code per customer. Unused credit expires on the date indicated on the media conveying the promotion code. The credit may be issued in increments as You use the credit over the period of time during which the credit is valid. If You do not use at least 50% of the credit issued within 3 months of the credit or credit increment being issued, Google may cancel all or part of the credit.
It seems that despite of expiration date being set in a year, Google can cancel it if I won't use it within 3 months.
To sum up: I would prefer to collect 12 credit codes in following 12 months and start using them when my app gains popularity and starts generating costs, but I am not sure if it is possible to wait that long (considering code expiration dates).