User groups are significantly different when there is a measurable difference in their experience with the design or domain area. For example, will you be testing expert vs. novice users? If so, then count those as separate use groups.

Different groups may also use the design for different purposes. For example, will you test system administrators vs. other users? If so, then count those as separate use groups.

User characteristics such as age ranges, gender, technology products owned, etc. usually do not constitute different groups. These qualities are often used to ensure diversity within a group, but not between groups.

If user groups will perform different tasks, then the number of participants needed will increase. In many cases, tasks are so different that it is akin to running separate studies. Consequently, the recruitment of participants should be treated as such.

For a baseline study, it is a good practice to increase the number of participants to increase the probability that most issues will be discovered.

Some usability studies compare two or more design alternatives to determine the best course of action for the overall product. The scope and severity of usability issues in each alternative are often compared to help in decision making.

For a comparative study it is a good practice to increase the number of participants to increase the probability that most issues will be discovered.

Answer "1" if each participant will see only one design (between subjects design) or greater than one if participants will see more than one design (within subjects design or repeated measures design).

Remember to consider practice effects, carryover effects, and participant fatigue.

Reliability of eye tracking and quantity of data is an issue. Start at 12 participants (for 10 to show) add 5 per user group (Blink experience).

Eye tracking as primary data requires more participants for statistically valid heat maps. Min 39 participants per design (Nielsen).