1. Can there be more than one mode?

Yes, if, for example there are three "5s" and three "6s" then the modes will be 5 and 6. This is said to be bimodal.

2. What is the disadvantage of using the median as an average for a set of scores?

It does not give any indication of any extremely small or large values in the set.

3. What is the disadvantage of using the mean as an average for a set of scores?

One really large or small value can have a big effect on the mean.

4. What is the disadvantage of using the mode as an average for a set of scores?

The mode may not be typical of the whole set of values. Could be a long way from the middle values of the set.

5. Which of the three averages mean, median and mode do people usually use?

The mean is probably what most people would normally think of as "the average".