From my own experience that number is cumulative. I regularly did Delves to level up brann when we had the DMF and or Hallows end up as he benefits from xp buffs (at a 20% reduction. So instead of 10% per buff, it is 8% per buff) and I was doing more than 28 in a week and was still getting xp. As for a long time I only did the 8 on my main for the vault then stopped.
iirc
/run print(C_CurrencyInfo.GetCurrencyInfo(3142).maxQuantity-C_CurrencyInfo.GetCurrencyInfo(3142).quantity)
is a script you can run that shows how many more bountiful delves you can do that will give xp for brann.