There is no clear-cut answer to this one. It is largely a matter of style. If the mystery shopping company has a style guide they ask shoppers to use when writing reports, that will determine how you use numbers. However, in the absence of specific style guidelines, here are some suggestions for when to use the numerals and when to spell out numbers.
Most people agree that the numbers one through nine should be spelled out.
There were six people in line ahead of me.
Some style guides insist on spelling one through ten, or even up to one hundred. There are exceptions to spelling out single-digit numbers, such as when you have a number less than ten and a number greater than ten in the same sentence. In that case, it may be preferable to use numerals for both:
There were 6 people in line ahead of me, and it took 14 minutes to reach the box office window.
Another example might be when giving timings, such as:
The hostess put me on hold for 1 minute and 32 seconds.
In fact, the mystery shopping company may prefer that you always use numerals when giving timings but that is, like all of these examples, an issue of style not correctness.
Because you want to avoid anything that would be hard to read or confusing, you may want to write out one of the numbers when two numbers are together in a sentence.
There were three 12-inch puddles of water in the entry.
Numbers at the beginning of a sentence should usually be spelled out.
Seventeen employees acknowledged me with a greeting.
If that starts to get ridiculous (e.g., a large number that is very long when written out), you can change the structure of the sentence. If you needed to tighten the above sentence, you could write:
I was greeted by 17 employees.
Remember that there are no hard and fast rules for when to use a numeral and when to write out the number. Make your writing clear and use a consistent style so that you are easily understood.