When someone in a restaurant kitchen says they need a certain number of dishes "all day," they are referring to how many dishes they need at that exact moment.
When servers deposit order tickets in the kitchen, it's the responsibility of the chef, sous chef, or other staff members to call out those orders in the most efficient way.
If seven different tables order salmon, it makes more sense to call out "seven salmon, all day," than it does to call out how many are needed for each individual ticket.
This also allows the kitchen team to assess in the moment if they have the ingredients to fulfill all of these orders, and if they anticipate running short on anything.
You might also hear "all day" alongside the term "fire." When a dish is fired, that means the server has input it into the system to generate a ticket or chit.