[semlar]

Here's a dry-coded example, so I have no idea if it works.

-- todo: Track that someone in the raid is actually capable of battle rezzing and alive
local WhoDead, SecondsPerRez, ElapsedTime, NumRezzed = {}, 1, 0, 0
local f = CreateFrame('frame') f:Hide()
f:SetScript('OnEvent', function(self, event, ...) return self[event] and self[event](self, ...) end)
 
function f:ENCOUNTER_START(encounterID, encounterName, difficultyID, raidSize)
    if difficultyID < 3 then return end
    wipe(WhoDead)
    SecondsPerRez, ElapsedTime, NumRezzed = 5400 / raidSize, 0, 0
    self:RegisterEvent('INCOMING_RESURRECT_CHANGED')
    self:Show()
end
f:RegisterEvent('ENCOUNTER_START')
 
function f:ENCOUNTER_END()
    self:Hide()
    self:UnregisterEvent('INCOMING_RESURRECT_CHANGED')
end
f:RegisterEvent('ENCOUNTER_END')
 
function f:INCOMING_RESURRECT_CHANGED(unit) -- Track who has received a rez within range
    local name = UnitName(unit)
    if name and not WhoDead[name] and UnitHasIncomingResurrection(unit) then
        WhoDead[name] = true
    end
end
 
f:SetScript('OnUpdate', function(self, elapsed)
    ElapsedTime = ElapsedTime + elapsed
    
    for name in pairs(WhoDead) do
        if not UnitIsDeadOrGhost(name) and UnitIsConnected(name) then -- Dead unit is no longer dead
            WhoDead[name] = nil
            NumRezzed = NumRezzed + 1
        end
    end
    
    local availableRezzes = 1 + floor(ElapsedTime / SecondsPerRez) - NumRezzed
    -- Do something to display this number somewhere..
end)

It doesn't actually do anything with the "availableRezzes" number, so if you want this for an LDB display or something you'll need to add the appropriate code for that.

I have no idea how reincarnation is factored into the new rules, and I don't know if reincarnation or soulstones trigger an incoming rez event, so it might just be simpler to check UNIT_HEALTH to track when they've died or rezzed.