You can build two more bridges. Once you achieve 100% approval (go to the town hall and talk to Isabelle for your % if you haven't gotten 100% yet) you'll be able to build
public works projects. Note that you won't be able to build most of these unless a villager suggests them to you. You can get one suggestion per day, and different villager personality types offer different suggestions. There's a link in the OP to a spreadsheet that has them all listed. If you want the different bridge PWPs to be suggested, you can only have two bridges at most up, otherwise they won't ever bring them up, for whatever reason.
You can't, but you can store items in a locker that can be accessed from any closet/dresser/etc or the train station locker. Later on you'll get a museum second floor which also has a locker. You can also store items in letters in your inventory.