Read the entry from an English usage guide.

Grammarians caution against using the words number and amount interchangeably. The general rule is to use number for countable objects (e.g., marbles) and amount for things that are not countable (e.g., effort). However, some grammarians suggest that it can be appropriate to use amount with countable nouns that are considered a unit or mass (e.g., the amount of tourists on the beach).

Based on the entry above, which sentence shows usage that would be MOST widely accepted?