how can i make a code snippet in vscode / Ultisnips(vim) that can have variable number of arguments (tab triggers) like this:

if i type input5, then it produces code like: cin >> $1 >> $2 >> $3 >> $4 >> $5;$0

and if i type input2, then it produces code like: cin >> $1 >> $2;$0

Another example: if i type arrinputmn (where m and n is the size of mxn matrix), then it produces code like:

${1:"int"} {$2:"arr"}[m][n] = {${3:"0"}};
for (int i = 0; i < m; i++) {
    for (int j = 0; j < n; j++) {
        int x;
        cin >> x;
        $2[i][j] = x;
    }
}

I want to know how i can access the code prefix (trigger name for snippet) and adjust the output code according to that

Sadly, that's not possible. Though you can get selected text which can create something similar to the second snippet request. The first you'll have to create a new snippet for each variation. Inconvenient, but not difficult.

From what I've seen you're not able to create logic from the snippet name, sadly. That is not a variable you have access to.

For the second, though again it can't be from the snippet name, you can get these numbers from the selected text if you're ok with that. This is fairly difficult and an advanced subject using regex. I have the answer though if you only have two variables, with the end output at the end.

If you're still willing to create a snippet that works a bit differently, the following in a way answers your second question. To confirm, the workflow would be writing the text _m_n (for example, if m=13 and n=52, you would write _13_52), selecting that text, then typing arrinputnm. Here's a gif of this behavior for clarity

As seen in this question, you can get the previous selection as a variable in your snippet.

using the variable "TM_SELECTED_TEXT"

As well as variable transforms in order to get only what we need we can create something useful.

In this situation we would want to type something like _12_34, select that text, then replace it with arrinputmn thus triggering our snippet

First, we need to find everything that is not the first occurrence of a number, and replace it with nothing. In regex this is /(_(?:[^_\\r]*)$|_)/g. or, fully, In VS Code variable transformation, this would be ${TM_SELECTED_TEXT/(_(?:[^_\\r]*)$|_)//g}

Secondly, we need to get replace the original underscore, any numbers, and then the second underscore. The regex for this being /(_\\d+_)// or, fully, ${TM_SELECTED_TEXT/(_\\d+_)//}

So, to conclude, the end output for your snippet would be this:

{
    "arrinputnm": {
        "prefix" :  "arrinputnm",
        "body": [
            "${1:int} ${2:arr}[${TM_SELECTED_TEXT/(_(?:[^_\\r]*)$|_)//g}][${TM_SELECTED_TEXT/(_\\d+_)//}] = {${3:0}};",
            "for (int i = 0; i < ${TM_SELECTED_TEXT/(_(?:[^_\\r]*)$|_)//g}; i++) { ",
                "\tfor (int j = 0; j < ${TM_SELECTED_TEXT/(_\\d+_)//}; j++) {",
                    "\t\tint x;",
                    "\t\tcin >> x;",
                    "\t\t$2[i][j] = x;",
                "\t}",
            "}"
        ],
        "description": "mxn matrix"
    }
}

Sorry I'm not able to get you the utilisnips output, I use luasnip which allows for textmate (VS Code) syntax in snippets.

If you want to do this again with another snippet the code ${TM_SELECTED_TEXT/(_(?:[^_\\r]*)$|_)//g} will get you the first number from your selection and the code ${TM_SELECTED_TEXT/(_\\d+_)//} will get you the second number in your selection