God bless you or God blesses you? Most of the time I heard people say God bless you.
It's "God bless you." It's a truncation of the imperative mode, "May God bless you." Of course if what you mean to say is that God is actually in the process of blessing someone, then, for example, "God blesses you for being so generous" would be correct. But people usually use this phrase as a plea to God to bless someone.
People also say "God help you," "God damn you," "God forgive you," etc. Even longer constructions are possible: "God bless the person who built this highway," "God forgive me for what I'm about to do," etc.
But we don't speak this way if the subject of the sentence is anyone but God. You have to use the auxiliary verb. "May your mother never know about this." "Let John recover from this illness."
God is singular or plural?
"God" is singular. It's a regular noun and the plural is "gods."
When used in a general sense, "god" is spelled with a lower-case G. "Thor is a Norse god." "The Egyptians had many gods." "Where is the god of poker when I need him?" "The gods are not smiling on me today." But when it's used in the context of the Abrahamic religions (Judaism, Christianity, Islam, Baha'i, Rastafari, Druze, etc.), since those people believe there is only one god, they consider it his name so they capitalize it, "God."
Footnote: 1. Even when speaking another language, Muslims usually use the Arabic word for god,
Allah, rather than its equivalent in the language they're using. 2. Rastafarians usually call him "Jah," which is a condensed form of one of the Hebrew words for god, "Yhwh," conventionally written in its Latin transcription "Jehovah" with arbitrary vowels inserted (for reasons much too complicated to go into here). 3. The Jews, ironically, don't often use that word except when reciting from the Torah in Hebrew. And they never say
Eloh, a cognate with the Arabic word--again unless reading from scripture, or discussing the gods of other religions. They just refer to theirs as God,
Dieu, Gott, Bog, etc., depending on the place where they live and the language they speak.