This is a question about how the usage of the terms maximum and highest.
First, the definitions (note that highest is the superlative form of high):
As great, high, or intense as possible or permitted
2 Great, or greater than normal, in quantity, size, or intensity
Suppose you are analysing 100 questionnaires, each with 3 questions Q1, Q2 and Q3, and the number of questionnaires returning 'o' scores for the questions were 20, 30 and 40 respectively.
Then you can say unambiguously that Q3 had the highest number of 'o' scores. In this context, highest refers simply to the score with the greatest size, irrespective of what that size could possibly be.
When it comes to maximum, it depends on the scope. If the context doesn't make clear that the scope is restricted to the 3 scores, then the phrase Q3 had the maximum number of 'o' scores would mean that Q3 had 100 'o' scores (100 is 'as great as possible' in this context). However, the context can restrict the scope, so you can also say that Q3 had the maximum number of 'o' scores out of the three questions.