Sorry, this is a tough, tough question. It depends on so many things, most especially in CBC:
- Number of attributes and levels per attribute in your study
- How many concepts you are showing per task
- Whether standard None is used (and how often respondents are clicking the None option) or Dual-Response None
- Whether first choice, chip allocation, or best-worst question format is used
- Whether respondents are answering using simple decision rules or more complex (compensatory tradeoff) decision rules
- And, it depends at least a little bit on sample size (since HB "borrows" information across respondents to stabilize estimates for each individual).
Please don't think there is some maximum of 20 recommended for CBC and 16 for MBC. While those seem like nice guidelines, it really depends on complexity of the survey, respondent motivation (incentives), etc.
Also, please be very careful about the "rule of thumb" involving "NTC >= 500". That formula was developed back prior to HB estimation, assuming analysis was only being conducted using aggregate counting or aggregate logit. So, that rule of thumb for sample size and number of tasks per respondent doesn't as directly apply to our modern-day situations involving HB.
I wish there were a good formula or rule of thumb for recommending the number of tasks and concepts per task to ask per respondent to lead to stable HB estimation. This requires a lot of experience, and depends greatly on the number of parameters you intend to estimate in the model (number of attribute levels in your study, and depending on if you need to estimate interaction effects).
All that said, many researchers lately are feeling that for standard CBC studies (involving about six attributes, each with about 2 to 5 levels) when showing 3 to 4 concepts per task, asking first choices only, and not requiring interaction effects...about 10 to 12 tasks is all that is needed for stable HB estimation. But, please be careful with this generalization! It can easily fall apart if some assumptions are violated.