Student or Learner
according to merit - definition of merit by Macmillan Dictionary
Merit ( an advantage or good quality that someone or something has ) is usually used in plural form
In what way can I use it in singular form with the same meaning ?
Does the meaning change if I use the singular form ?
The link you provided gives examples of sentences containing the singular form. The 'meaning' does not change in singular and plural, though the number does.