"All" needs to modify a plural noun, so "intents and purposes" would be correct.
Could you rewrite your question as a full sentence, please? It's not clear what you are asking.
"For all intents and purposes" is the correct phrase according to American usage. The British version of the idiom is "To all intents and purposes." (See the Related link.)
The correct saying is "all intents and purposes".
Question is its own answer, like the sound of one hand clapping