No. Singular or plural nouns have no effect on the use of "my" or "mine", but rather the word order.
"This is my apple" and "These are my apples".
"This apple is mine" and "These apples are mine".
Of course, sorry. Although your original question was in English, I see that the text in question was in Danish, so the answer should be:
The plural of "tøj" is "tøj", and "tøjet" is the word for "the clothes", and therefore "My clothes" translates as "Mit tøj".
Another word for clothes is, "Klæder", and here one would use "Mine klæder" for "My clothes".
No, in the first link, you can see that the plural of tøj is tøjer. After BØJNING the endings -et, -er, -erne appear, i. e. the definite singular is tøjet, the indefinite plural is tøjer, and the definite plural is tøjerne. If the plural of tøj was tøj the endings would be -et, -, -ene.
In reply to your comment 5 hours before this one: That is what appears in the dictionary, but is in contradiction to the statement further down the page, under "betydning/grammatik". I have never ever heard the word "tøjer" or "tøjerne" used in the 51 years i have lived in Denmark. Perhaps you could enlighten on the subject by giving an example of when you would use "tøjer" as the plural for clothes.
J.C.M.H. - Further to my comment of 9 hours ago: It is strange that the same dictionary gives two contradicting explanations for the plural of "tøj", so I have searched a little more and found yet another link. According to "Glosbe", "tøjer" is the plural indefinite of "tøj", and it continues to give a very comprehensive list of examples where "tøjer" and "tøjerne" would be used. To add to the confusion, ALL examples mention no name of "tøjer" or "tøjerne", but simply "tøj" in all cases. I'll comment again if I find a more clear explanation.
No. The plural of "tøj" is "tøj". The word "tøje (-er) is an old word for an elongated tot or trille formed by kartning. In the first link, under "betydning/grammatik", you will read that there is no pluralis for "tøj".