The problem isn't really with "subquestions" in total, the problem appears with subquestions in multipe answer questions. Because for these questions one column per answer option / subquestion needs to be reserved. Should you have 50 questions, all with array or single choice questions, the number of subquestions should not be a problem.
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Note: I answer at this forum in my spare time, I'm not a LimeSurvey GmbH employee.