Well, the simple answer is just to add as many tasks such that each level appears as many times as it would have under a full-profile study.
For example, imagine that you had a 8-attribute full-profile study that you were comfortable asking 10 tasks for to obtain enough information at the individual level. If you decided to do a partial-profile approach showing just 4 of the 8 attributes in each task, then you know each task contains half has much information. So, to compensate, you double the number of tasks to 20 to achieve the same amount of information as you had before.
Keith Chrzan has done research showing that the response error for partial-profile tasks is lower than for full-profile, so Keith would argue you don't need double as many tasks in the example above because the loss in sheer information for PP is counteracted by the reduction in respondent error. What exactly the tradeoff is, however, I don't know.