A media item (MediaItem*) contains one or more takes (MediaItem_Take*).
Code:
for ii=0, reaper.CountSelectedMediaItems(0)-1 do
local item = reaper.GetSelectedMediaItem(0, ii)
-- go through all takes
for ti=0, reaper.CountTakes(item)-1 do
local take = reaper.GetMediaItemTake(item, ti)
reaper.TakeFX_GetCount(take)
end
-- or just the current take
local take = reaper.GetActiveTake(item)
reaper.TakeFX_GetCount(take)
end