I’m unsure about a line in the hymn “Count Your Many Blessings”

Answer:

The fourth verse of Count Your Many Blessings by Oatman Johnson reads:

"So, amid the conflict, whether great or small,
Do not be discouraged, God is over all;
Count your many blessings, angels will attend,
Help and comfort give you to your journey's end."

Hymns are not inspired writing so there can be errors, but generally old hymns have stood the test of time. If there were an error, you would suspect that it would have been noticed before.

As you pointed out, in discussing angels, the writer said, "Are they not all ministering spirits, sent out to render service for the sake of those who will inherit salvation?" (Hebrews 1:14). The fact that angels are sent to render service is mentioned in other places as well.

"The angel of the LORD encamps around those who fear Him, and rescues them" (Psalms 34:7).

"For you have made the LORD, my refuge, even the Most High, your dwelling place. No evil will befall you, nor will any plague come near your tent. For He will give His angels charge concerning you, to guard you in all your ways. They will bear you up in their hands, that you do not strike your foot against a stone. You will tread upon the lion and cobra, the young lion and the serpent you will trample down. Because he has loved Me, therefore I will deliver him; I will set him securely on high, because he has known My name. He will call upon Me, and I will answer him; I will be with him in trouble; I will rescue him and honor him. With a long life I will satisfy him and let him see My salvation" (Psalms 91:9-16).

We have examples of Daniel, Jesus, and Peter all being protected from danger. We have an example of Hagar being encouraged (Genesis 21:17).

Songwriters are restricted in the number of words and the type of words they can use to describe the concepts they want to get across. Yes, Hebrews 1:14 doesn't give details but that doesn't mean it is the only passage that we can examine.