From WoWDev

ItemSet.dbc

This client database lists the various item/armor sets.

Fields

Field ID	Type	Description
0	uint	ItemSetID
1	string	Name
10-17	uint[8]	ItemID list
66-73	uint[8]	SpellID list
74-81	uint[8]	ItemCount list

Field descriptions

Each item set can contain up to 8 items. Each Item Set can have up to 8 spells which are activated depending on the number of items for this ItemSet you currently have. So in short there are 8 SpellID fields which reference a spell in the Spells.dbc. For each SpellID there is a corresponding ItemCount field. The combination of those two gives us for example that SpellID 01 gets activated if you have ItemCount01 items of the given ItemSet.

For Example

ItemSetID	ItemSetName	<snipped>	ItemID01	ItemID02	<snipped>	SpellID01	SpellID02	<snipped>	ItemCount01	ItemCount02
186	Beaststalker Armor	...	16680	16675	...	20877	7569	...	3	5

This means that the BeastStalker Armor set consists of items 16680, 16675, ... It also shows us that spell 20877 gets activated if you have 3 pieces of the BeastStalker armor, and spell 7569 gets activated if you have 5 pieces of BeastStalker Armor.