The Prose Edda 1.40

Then said Gangleri: "These are marvellous tidings which thou now tellest. A wondrous great house Valhall must be; it must often be exceeding crowded before the doors." Then Hárr answered: "Why dost thou not ask how many doors there are in the hall, or how great? If thou hearest that told, then thou wilt say that it is strange indeed if whosoever will may not go out and in; but it may be said truly that it is no more crowded to find place therein than to enter into it; here thou mayest read in Grímnismál:

Five hundred doors and forty more So I deem stand in Valhall; Eight hundred champions go out at each door When they fare to fight with the Wolf."