When we are making nouns plural, we use "-lar/-ler". It is like using, "-s/-es-ies" suffices, in English. Why do we say "TeşekkürLER" instead of "Teşekkür"? In some languages, idioms are formed this way. In English, we say "ThankS", not "Thank". It is also similar to Turkish, teşekkürLER. However in French we say "Merci" and it is not plural. Thus, I believe there is no logical explanation to resolve all the usages for all the languages, we have to memorize idioms for each language. Apart from that, in Turkish we dont have suffices to make plural. Your examples indicate being polite. The subject of the sentence changes, like conjugation. Hoşgeldin (sen, 2nd person singular) Hoşgeldiniz (siz, 2nd person plural) Ben (I) Sen (You) O (He/She/It) Biz (We) Siz (You) Onlar (They) Now, when comparing with English, even if we talk polite or impolite, we use "you" and conjugate the same. Turkish is not like that. When we address someone in a polite way, we use 2nd person plural (siz). I can say that it is similar to French, tu and vous. What is your name? (normal form) Adın ne? What is your name? (polite form) Adınız ne? Name is not plural. There is still only one name. And actually there is also a single man to whom we ask the question, however we are being polite and we address him "siz", "sizin adınız"

Actually ''hoş geldin'' exists and it's being used very often. ''Hoş geldiniz'' is being used for a group of people or when you want to greet formally. Even in English one says ''thanks'' and not ''thank'' so I would say ''teşekkürler'' sounds pretty normal. ''İyi günler/akşamlar/geceler'' are plural but I don't think there is a reason for it.