It depends on the information on that sheet. If it is just a sheet with project information like project name and address, then maybe it could go without a number but if it has any information about the drawing set or any type of references necessary to build the project then it definitely needs a number.
In other words, if it is just a fancy sheet with a couple logos and maybe a picture or two that acts as a cover/protector for the true drawing set, like the cover a book, then you could go with no number. Anything more than that, it definitely needs a number.