I've heard that there are some chapters in shadow dragon that can only be accessed if you have a certain number of units or less, which usually means killing off units. how many units must you have lost to get these chapters, if there are any?
Based on the wiki list of chapters, there a total of 5 chapters that can only accessed based on the available units. These chapters are noted as "Chapter NX"
To advance to this chapter, the number of alive units must be 15 or less by the end of Chapter 6.
To gain access to this chapter, complete Chapter 12: The Ageless Palace with 15 units or less.
To gain access to this chapter, finish Chapter 17 with 15 units or fewer alive.
To advance to this chapter, complete Chapter 20: Camus the Sable with 15 or fewer units alive
If you want to advance to this Gaiden Chapter you must have Tiki dead (or not recruited) and the Falchion not in possession by the end of chapter 24