If you want Core to create new projects with their IDs in sequence automatically, you can do that using the option Auto increment Project ID in Global Settings > Master Information. After you check this option, enter one number less than the starting number you want to use for the sequence in the ID field under Project ID below this option and save the changes. Now when you create a new project, its Project ID is auto-filled with the first number of your sequence.
For example, if you want the sequence to start at 101, enter 100 in the ID field and create a new project. The new project has the ID 101 auto-filled. Note that the ID field in Global Settings also increments by 1 after you create the new project, so that at any time, when you create a new project, its ID will be one more than the current value of this field, leading to a sequence of Project IDs. Check the Core Help for more details.