[Dignity]

Use an array.

Example (obviously this might not work and isn't the most efficient example, but it should give you an idea on how to do it):

pawn Code:

#include <a_samp>

#define RAND_DIALOG0     0
#define RAND_DIALOG1     1
// etc

new bool: pDialogUsed[MAX_PLAYERS][5];

public OnPlayerCommandText(playerid, cmdtext[])
{
    if (strcmp("/randomdialog", cmdtext, true, 10) == 0)
    {
        jump:
    
        switch(random(5))
        {
            case 0:
            {
                if(pDialogUsed[playerid][0] == false)
                {
                    ShowPlayerDialog(playerid, RAND_DIALOG0, DIALOG_STYLE_MSGBOX, "Insert Info Here", "Random Dialog 0", "Select", "Select");
                }
            
                else goto jump;
            }
            
            case 1:
            {
            // etc
            }

        }

        return 1;
    }
    
    return 0;
}

public OnDialogResponse(playerid, dialogid, response, listitem, inputtext[])
{
    if(dialogid == RAND_DIALOG0)
    {
        if(response || !response)
        {
            pDialogUsed[playerid][0] = true;
            // do some stuff
        }

        return 1;
    }

    return 0;
}

Don't expect fully working code neither, this is a sub section to ask help with scripting, not for people to make your scripts for you.