[Garkin]

This code probably looks a bit better:

local bossCount = 0
local bossUnitTags = {}
for i = 1, MAX_BOSSES do         --global constant MAX_BOSSES = 6
   bossTags[i] = "boss" .. i
end
 
local function OnBossesChanged()
   bossCount = 0
   for i, bossTag in ipairs(bossUnitTags) do
      if DoesUnitExist(bossTag) then
         bossCount = bossCount + 1
      end
   end
end
 
EVENT_MANAGER:RegisterForEvent("MyAddon", EVENT_BOSSES_CHANGED, OnBossesChanged)
 
--returns true if unit is boss
function IsUnitBoss(unitTag)
   if bossCount > 0 and unitTag ~= nil then
      for i, bossTag in ipairs(bossUnitTags) do
         if GetUnitName(unitTag) == GetUnitName(bossTag) then
            return true
         end
      end
   end
   
   return false
end