During the introduction sequences to every mission, a number that corresponds to that mission can be found.

Mission 1: A big "1" on the pizza box.
Mission 2: A "2" on a billboard near the outside of Dante's shop.
Mission 3: Three enemies are burned by Lady's motorcycle and fall to make a "3".
Mission 4: There is a banner near where Vergil and Arkham are standing in the shape of a "4".
Mission 5: A bloodgoyle gets hit into a wall. The blood remaining on the wall is in the shape of a "5".
Mission 6: Lady blasts a rocket into the ground and the mark on the ground forms a "6".
Mission 7: After Lady shoots Dante in the head, he smears the blood on the wall. The blood runs into the shape of a "7".
Mission 8: When Dante gets up, goes near the pillar, and is about to jump off, you can see a faint shape of an "8" in the moon behind him.
Mission 9: One of Lady's spent shells falls on the ground. A "9" is on the bottom of the bullet.
Mission 10: When Vergil stabs Arkham the blood spatters Arkham's book in the form of a "10".
Mission 11: When Arkham disappears from where he was stabbed, the smeared blood leaves an "11" behind.
Mission 12: After Dante grabs the Haywire Neo-Generator, Jester escapes on something shaped like a "12".
Mission 13: Immediately after Vergil cuts his hand to make a sacrifice, the camera angle changes. While the angle is turning you can see a "13" carved into a pillar.
Mission 14: At the end of the intermission sequence, it shows an angle aiming down into the pit where Vergil fell. On the wall of the crevice is a carved "14".
Mission 15: After awhile after the intermission sequence starts, the camera will face a door with three symbols on it. Just to the right of it is a dried up plant. The branches are shaped into the number "15".
Mission 16: A little while after Dante starts changing between normal and demon forms he will take out his guns and shoot them at the same time. If you can spot it, the flash of the two gunshots makes "16". You must watch extremely carefully.
Mission 17: After Arkham pulls the force edge from the stone, the rocks shift around. In the bottom left of the screen will be two rock pillars next to each other. They form a "17".
Mission 18: As Dante is rising up the portal to the demon world, a single newspaper flies around the bottom right corner of the screen with an "18" on it.
Mission 19: The camera will start to turn at the beginning of the intermission sequence. As it is turning, there is a bookshelf. There is an area with books missing in the shape of a "19".
Mission 20: After Lady shoots her father and starts crying in the background, the mist forms the number "20".