Break out _count_data_items() function and implement #5102

Job	Run time
Shellcheck	3s
Dependency	7s
Prepare	58s
Ruff check and force	6s
Check commit	0s
Create pytest cache for Python ${{ matrix.python-version }}	0s
Run pytest using Python ${{ matrix.python-version }}	0s
Run mypy	0s
Process test coverage	0s
Build and publish Python 🐍 distributions 📦 to TestPyPI	0s
Process test complexity	0s
	1m 14s

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