According to https://biblehub.com/strongs/2_thessalonians/2-14.htm
In the phrase, "He called you...", the "you" is ὑμᾶς, which is "Personal / Possessive Pronoun - Accusative 2nd Person Plural".
So if it is plural, why does Strong define it as "You. The person pronoun of the second person singular; thou"?
Let us begin with a literal translation of 2 Thessalonians 2:14 -
to this also He called you through our gospel for obtaining of glory of our Lord Jesus Christ.
As the OP has already pointed out, "you" is is translated from ὑμᾶς (humas) which is Personal Possessive Pronoun - Accusative 2nd Person Plural
Strong's defines this word as:
The person pronoun of the second person singular; "thou".
Strong's does this because this is the "lexical form". That is, "thou" is nominative second person singular, and the second person plural is "you" (in English this is both nominative and accusative). The possessive or genitive is "your".
We see the same "lexical language" just two words further on with ἡμῶν (hēmōn) which is correctly listed by Strong's as "Personal / Possessive Pronoun - Genitive 1st Person Plural" [i.e. "our"] of "the first-person pronoun, I".
That is, Strong's is a lexicon and uses lexical language.
BibleHub has indexed it wrongly. It should be 5209. The Greek word ςύ is the personal pronoun and can be either singular or plural. Though, if you're not aware, the singulars and plurals look nothing alike.
However, historically, Strong's is a concordance and really shouldn't be thought of as a lexicon. It certainly does not provide definitions. I would encourage you to pursue more robust tools that give definitions. The newer lexicons have moved that direction. What glosses are doing is nothing more than giving hints to meaning by listing how the word has been translated. This is misleading because words only have meaning in context. So, definitions provide a lexical semantic basis where glosses require context. Strong's effectively rips the word out of its context.
An example of the difference is this (from Danker's Concise Lexicon):
ἐπιτριπή: 'right to act officially'
versus
ἐπιτριπή: empowerment, full power, commission.
Readers of the Bible in English will find it useful to refer to THE ENGLISHMAN'S GREEK CONCORDANCE by George V. Wigram which has been Numerically Coded to Strong's Exhaustive Concordance. The first edition was published by Samuel Bagster & Sons in eight volumes in 1839. To the Ninth Edition of 1903 was Added "A Vocabulary of New Testament Greek". Baker Book House added "Index of Greek Roots Not Used in the New Testament", and Numerically Coded this book to Strong's Exhaustive Concordance. Copyright 1979 by Baker Book House Company. A companion Volume is THE ENGLISHMAN'S HEBREW AND CHALDEE CONCORDANCE OF THE OLD TESTAMENT originally published in 1843 by Samuel Bagster & Sons, London. The fifth edition, used in the preparation of the Edition by Baker Book House Company, copyright 1980, was coded with Strong's Exhaustive Concordance and published in 1890 by Samuel Bagster & Sons, London. When you read the text of the Holy Bible please read the full verse, and also read the preceding verse and the following verse. The context of the sentence you are referring to will usually clearly answer and explain your query in clear English. Wigram links 2 Thessalonians 2:14 "Whereunto he called YOU by our gospel" to the Greek word humas, numbered 5209.
