Fix off-by-one expectation and simplify partitioning specs a little

What does this MR do?

This adds an expectation towards the total number of partitions per table and simplifies the expectation a little. Suggestion by @pbair in !34652 (9ab1daa5, comment 362902228)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
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• [-] Separation of EE specific content