This adds a label to the rightSlot so you can count your ammo.
This is only an example but it works for burst arrows.

local ammoLabel = nil
macro(200, "count", function()
    local count = 0
    if getRight() then
        if getRight():getId() == 35849 then -- red quiver
            count = player:getItemsCount(3449) -- burst arrow
        elseif getRight():getId() == 35848 then -- blue quiver
            count = player:getItemsCount(3448) -- poison arrow
        end
    end
    ammoLabel:setText(count > 0 and count or "")
end)

quiverSlot = modules.game_inventory.inventoryWindow:recursiveGetChildById('slot5')
ammoLabel = quiverSlot:getChildById("AmmoLabel")

if not ammoLabel then
    ammoLabel = g_ui.loadUIFromString([[
Label
  color: #5ff7f7
  background-color: #00000055
  opacity: 0.87
  anchors.left: parent.left
  anchors.right: parent.right
  anchors.bottom: parent.bottom
  text-align: right
  margin-right: 3
  margin-left: 3
  text:
]], quiverSlot)
    ammoLabel:setId("AmmoLabel")
end