The text of the Flaming Sphere spell says that "If you ram the sphere into a creature, that creature must make the saving throw against the sphere's damage".
Can I ram a Flaming Sphere into multiple tiny creatures, if they occupy the same 5' by 5' square?
I see nothing forbidding the sphere from being simultaneously rammed into more than one creature.
The wording of the spell indicates a single creature. Once it rams into a single creature, the sphere stops. As such, it can only hit one creature before stopping.
As a bonus action, you can move the sphere up to 30 feet. If you ram the sphere into a creature, that creature must make the saving throw against the sphere's damage, and the sphere stops moving this turn.
The others will take damage if they are 5' of the sphere when they end their turn.
The wording is "If you ram the sphere into a creature". The ability to ram the sphere is assumed to exist outside of the spell description, and is not defined explicitly by the spell.
I'd argue that if you move a sphere into 2 tiny creatures, both are simultaneously rammed by the sphere, and the condition "if you ram the sphere into a creature" is triggered twice.
