Getting input from a player for an API script and calling a macro from a script

The basics of creating an API command is the chat:message message event. You'll have a callback function with one parameter, which will be an object storing all the info you need. the 'content' property stores the original text of the message.

on('chat:message', function(msg) {
    if (msg.type !== 'api') return; // type will be 'api' if the user began the message with "!", making it an API command

    var parts = msg.content.split(' '); // msg.content is the text of the message
    var command = parts.shift().substring(1); // shift() removes the first element of the array and returns it

    if (command.toLowerCase() === 'repeatroll' && parts.length > 1) {
        var roll = parts.slice(0, parts.length - 1); // slice() is like substring() for arrays
        var repeat = parseInt(parts.pop()); // pop() removes the last element of the array and returns it
        if (!repeat) return; // if the parseInt() returned NaN, quit

        var output = '';
        for(var i = 0; i < repeat; i++) {
            output += '[[' + roll + ']], ';
        }
        output = output.substring(0, output.length - 2);

        sendChat('player|' + msg.playerid, output); // player|-Jacb123 posts a message as the player with the id -Jabc123,
                                                    // character|-J123abc posts a as the character with the id -J123abc,
                                                    // any other value for the first parameter is like using /as
                                                    // playerid is the id of the player that send the API command
    }
});

The above is pretty rough and dirty, but it should be a functional solution to what you're looking for. You'd have a macro like !repeatroll 1d20 + 5 10 and it would roll 1d20+5 10 times. With some programming background, you should be able to make sense of the above pretty easily.

If you want something more elegant, HoneyBadger's Power Cards script accomplishes what you're looking for and a whole lot more. It's really quite a good bit of work, although it's long, complicated, and hard to read for someone just jumping into the world of Roll20 API Scripts.

on('chat:message', function(msg) {
    if (msg.type !== 'api') return; // type will be 'api' if the user began the message with "!", making it an API command


    var parts = msg.content.split(' '); // msg.content is the text of the message
    var command = parts.shift().substring(1); // shift() removes the first element of the array and returns it


    if (command.toLowerCase() === 'rr' && parts.length > 1) {
        var roll = parts.slice(0, parts.length - 1); // slice() is like substring() for arrays
        var repeat = parseInt(parts.pop()); // pop() removes the last element of the array and returns it
        if (!repeat) return; // if the parseInt() returned NaN, quit


        var output = '&{template:default} {{name=Damage Roll}}';
        for(var i = 0; i < repeat; i++) {
            output += " {{[[" + roll + "]] Damage to =[[(1t[Hit-Location])]]}}";
        }
        sendChat('player|' + msg.playerid, output); // player|-Jacb123 posts a message as the player with the id -Jabc123,
                                                    // character|-J123abc posts a as the character with the id -J123abc,
                                                    // any other value for the first parameter is like using /as
                                                    // playerid is the id of the player that send the API command
    }
});

output += " {{[[" + roll + "]]=[[(1t[Hit-Location])]]}}";

on('chat:message', function(msg) {
    if (msg.type !== 'api') return; // type will be 'api' if the user began the message with "!", making it an API command


    var parts = msg.content.split(' '); // msg.content is the text of the message
    var command = parts.shift().substring(1); // shift() removes the first element of the array and returns it


    if (command.toLowerCase() === 'rr' && parts.length > 1) {
        var roll = parts.slice(0, parts.length - 1); // slice() is like substring() for arrays
        var repeat = parseInt(parts.pop()); // pop() removes the last element of the array and returns it
        if (!repeat) return; // if the parseInt() returned NaN, quit
        
        var location;
        
        var output = '&{template:default} {{name=Damage Roll}}';
//        var output = "[[" + roll + "]] Damage to [[1t[Hit-Location]]]";
        for(var i = 0; i < repeat; i++) {
            var tableRoll = randomInteger(100);
            if(tableRoll <=50) location = "Body";
            else if(tableRoll <=60 && tableRoll > 50) location = "Head";
            else if(tableRoll <=70 && tableRoll > 60) location = "Left Arm";
            else if(tableRoll <=80 && tableRoll > 70) location = "Right Arm";
            else if(tableRoll <=90 && tableRoll > 80) location = "Left Leg";
            else if(tableRoll <=100 && tableRoll > 90) location = "Right Leg";
            else location = "Error";
            output += " {{[[" + roll + "]] Damage to="+location+"}}";
        }
        sendChat('player|' + msg.playerid, output); // player|-Jacb123 posts a message as the player with the id -Jabc123,
                                                    // character|-J123abc posts a as the character with the id -J123abc,
                                                    // any other value for the first parameter is like using /as
                                                    // playerid is the id of the player that send the API command
    }
});