'Researches' can be used. My researches into ..... I'm going to make researches into.... Their researches were not successful. It may not be as common as the singular form but it is used, and it is correct. Both 'research' and 'researches' can follow 'all.' All research was stopped. All researches were fruitless. The Free Dictionary online gives the example- "Soon after, Faraday began his researches into electricity." If you changed that into "... Faraday began his research into electricity," you would actually suggest it was perhaps one more continuous and coherent piece of research. By using the plural it suggests many lines of research, perhaps. So, in summary, research is often used in the singular but can mean a lot of different research! If you wish to use researches because it suggests the plurality of projects more... then you can do so. Often we use researches after a possessive form such as My, His, Fred's Fred's researches Faraday's researches

"Research" is a non-count noun. There is no plural form.

I believe that Americans may have dropped the use of the word 'researches' and prefer to use just 'research' as a noun. But as I am English I am not an expert on the changes American have made. It actually makes sense to use just research as an uncountable noun and leave researches as a verb. But in English English 'researches' is fine. However, my researches into this subject will continue.

Some nouns in English do not have a plural form or, if they do, the plural form is not used. "Research" is one of these. I cannot think of an example of "researches" as a plural of "research". One would not say "my researches in this field". One would say "My research into this field" Similarly, "our research", "recent research", "all research", etc. If it's important to use a plural noun, one would use "research" as an adjective e.g. "research studies", "research investigations", etc.

Think of it this way. "Research" is a type of activity. No matter how many people are doing it, it is still the same singular activity. The same will hold true for other activitiies such as: homework, singing, soccer, work, etc.