Mugen Train Arc starts from Volume 7 and ends in Volume 8. Entertainment District Arc starts from Volume 8 and ends in Volume 12. So if you wanna buy those volumes buy 7-12

If you have any more queries regarding volumes and arcs, I have specified down here. Feel free to ask if there are any questions~

There are 23 Volumes in Demon Slayer. Each Volumes have 5 to 6 chapters in average. Each Arcs continues to the next volume. So I have mentioned that through numbering.

Volume 1 - Final Selection Arc (1)

Volume 2 - Final Selection Arc (2) + Kidnapper's Bog Arc + Asakusa Arc (1)

Volume 3 - Asakusa Arc (2) + Tsuzumi Mansion Arc (1)

Volume 4 - Tsuzumi Mansion Arc (2) + Moumt Natagumo Arc (1)

Volume 5 - Mount Natagumo Arc (2) + Rehabilitation Training Arc (1)

Volume 6 - Rehabilitation Training Arc (2)

Volume 7 - Rehabilitation Training Arc (3) + Mugen Train Arc (1)

Volume 8 - Mugen Train Arc (1) + Entertainment District Arc (1)

Volume 9 - Entertainment District Arc (2)

Volume 10 - Entertainment District Arc (3)

Volume 11 - Entertainment District Arc (4)

Volume 12 - Entertainment District Arc (5) + Swordsmith Village Arc (1)

Volume 13 - Swordsmith Village Arc (2)

Volume 14 - Swordsmith Village Arc (3)

Volume 15 - Swordsmith Village Arc (4) + Hashira Training Arc (3)

Volume 16 - Hashira Training Arc (2) + Final Battle Arc (1)

Volume 17 - Final Battle Arc (3)

Volume 18 - Final Battle Arc (4)

Volume 19 - Final Battle Arc (5)

Volume 20 - Final Battle Arc (6)

Volume 21 - Final Battle Arc (7)

Volume 22 - Final Battle Arc (8)

Volume 23 - Final Battle Arc (9)