Article before the word “additional”

It is known that an indefinite article may be used before the word “additional” followed by a plural noun.

Example: An additional 5 sheets were used.

But phrases like “additional resources” (without an article) are also possible. Does it mean that an indefinite article should be used only when the word “additional” is followed by a determiner and a plural noun?

Answer

Generally speaking, yes. You are correct. You would always say things like:

An additional 10 units....

An added 50 barrels....

or

Additional units were needed.

Added barrels increased the total volume.

It's not necessarily the case for every single word or condition. But in general, and definitely for the word in question (additional), that is the rule.