I just got the soundtrack in the mail yesterday. 72 songs, 36 on each disc. At least two songs per character-franchise, except for Mega Man and Sonic who didn't get any. The Blue disc also has the currently unreleased
Mario Paint Medley, which will play on the Miiverse stage. A few of the songs included are from Melee and Brawl, but most of the music was created for Smash 4. The Red disc plays songs that are found in the 3DS version while Blue sticks to Wii U music.
Also it's entirely in Japanese, including the version of Ashley's Song from Warioware. Every song label and artist is in Japanese so if you want to rip them, be ready to rename everything.
The complete song list:
Red
1: Menu
2: How to Play (Melee)
3: Overworld (Super Mario Bros)
4: Overworld/Aquatic (Super Mario 3D Land)
5: Try and Try Again (Mario & Luigi Superstar Saga)
6: Kongo Jungle (Melee)
7: Stickerbrush Symphony (DKC2)
8: Gerudo Valley (Zelda OoT)
9: Overworld (Zelda Spirit Tracks)
10: Brinstar (Melee)
11: Obstacle Course (Yoshi's Island)
12: Green Greens V.2 (Kirby's Dream Land)
13: Corneria (Melee)
14: Star Wolf/Sector Z (Star Fox 64)
15: Battle! Vs. Trainer (Pokemon X/Y)
16: N's Castle (Pokemon Black/White)
17: Mute City V.3 (F-Zero)
18: Id (Purpose) (Fire Emblem Awakening)
19: Wrath of the Reset Bomb (Kid Icarus Uprising)
20: Dark Pit's Theme (Kid Icarus Uprising)
21: Ashley's Song (JP) (Warioware Touched)
22: Tortimer Island (Animal Crossing)
23: Kappn's Song (Animal Crossing)
24: Jogging/Countdown (Punch-Out)
25: Gaur Plains (Xenoblade Chronicles)
26: You Will Know Our Names (Xenoblade Chronicles)
27: Balloon Fight Medley (Balloon Fight)
28: Bathtime (Vocal Mix) (nintendogs)
29: Save the World, Heroes! (Find Mii 2)
30: Tomadachi Life (Tomadachi Life)
31: Pac-Man Club Mix (Pac-Man)
32: Tetris Type A (Tetris)
33: Trophy Rush
34: Multi-Man Smash
35: Credits
36: Practice Stage
Blue
1: Battlefield
2: Super Mario Bros. Medley (Super Mario Bros.)
3: Athletic Theme (New Super Mario Bros.)
4: Egg Planet (Super Mario Galaxy)
5: Mario Circuit (Mario Kart 8)
6: Gear Getaway (Donkey Kong Country Returns)
7: Title (Vocal Mix) (Donkey Kong Country Returns)
8: Ballad of the Goddess/Girahim's Theme (Zelda Skyward Sword)
9: Overworld Theme (The Legend of Zelda)
10: Ocarina of Time Medley (Zelda OoT)
11: Temple (Melee)
12: Title (Metroid)
13: Ending (Yoshi's Story)
14: The Great Cave Offensive (Kirby Super Star)
15: The Legendary Air Ride Machine (Kirby's Air Ride)
16: Area 6/Missile Slipstream (Star Fox 64/Command)
17: Battle! Team Flare (Pokemon X/Y)
18: Battle! Champion Cynthia (Pokemon Diamond/Pearl)
19: Route 10 (Pokemon Black/White)
20: Fight 1 (Fire Emblem Gaiden)
21: Destroyed Skyworld (Kid Icarus Uprising)
22: World Map (Pikmin 2)
23: Xenoblade Chronicles Medley (Xenoblade Chronicles)
24: Plaza/Title (Animal Crossing)
25: Tour (Animal Crossing New Leaf)
26: Duck Hunt Medley (Duck Hunt)
27: Light Plane (Vocal Mix) (Pilotwings)
28: The Mysterious Murasame Castle Medley (The Mysterious Murasame Castle)
29: Mii Plaza (Mii Maker)
30: Mario Paint Medley (Mario Paint)
31: Pac-Man's Park/Block Plaza (Pac-Mania)
32: Pac-Man (Pac-Man)
33: Master Hand
34: Credits V.2 (Super Smash Bros. 64)
35: Menu V.2 (Super Smash Bros. Melee)
36: Results Screen (Super Smash Bros. 64)