If you just want to actually test what you said it should be enough to check for Input.anyKey

Is any key or mouse button currently held down

if(Input.anyKey)

If you want to check specific keys you could use Input.GetKey and Linq Count like e.g.

// Fancy queries ;)
using System.Linq;

...

// The keycodes you wan to check
private HashSet<KeyCode> keysToCheck = { KeyCode.W, KeyCode.A, KeyCode.S, KeyCode.D };

int numberOfKeysPressed;

private void Update ()
{
    numberOfKeysPressed = keysToCheck.Count(key => Input.GetKey(key));
    // This basically equals doing 
    // var numberOfKeysPressed = 0;
    // foreach(var key in KeyToCheck)
    //{
    //    if(Input.GetKey(key)) numberOfKeysPressed++;
    //}
}

Or you could use Input.GetKeyDown and Input.GetKeyUp and do something like

private HashSet<KeyCode> keysToCheck = { KeyCode.W, KeyCode.A, KeyCode.S, KeyCode.D };

int numberOfKeysPressed;

private void Update ()
{
    foreach (var key in KeyToCheck)
    {
        if(Input.GetKeyDown(key)) numberOfKeysPressed++;

        if(Input.GetKeyUp(key)) numberOfKeysPressed--;
    }
}

If you want all KeyCode values you could use

private void Awake ()
{
    keysToCheck = new HashSet<KeyCode>((KeyCode[])Enum.GetValues(typeof(KeyCode)));
}

If you really want to get the amount of any key presses on the keyboard it would probably be more efficient to directly use Event.current and do something like

HashSet<string> currentlyPressedKeys = new HashSet<string>();

void OnGUI () 
{
    Event e = Event.current;
    switch(e.type)
    {
        case EventType.KeyDown:
            var key = e.keyCode.ToString();
            if(!currentlyPressedKeys.Contains(key)) currentlypressedKeys.Add(key);
            break;

        case EventType.KeyUp:
            var key = e.keyCode.ToString();
            if(currentlyPressedKeys.Contains(key)) currentlypressedKeys.Remove(key);
            break;
    }
}

Now wherever you need to know you can do e.g

if(currentlyPressedKeys.Count > XY)

I dont think theres is a specific method for this.

You could however write your own counter method using Input.GetKeyDown.

Like so

int GetKeysDownCount() {
    var keysDown = 0;
    if(Input.GetKeyDown(KeyCode.W)
        keysDown++;
    if(Input.GetKeyDown(KeyCode.A)
        keysDown++;
    if(Input.GetKeyDown(KeyCode.S)
        keysDown++;
    if(Input.GetKeyDown(KeyCode.D)
        keysDown++;
    return keysDown;
}

https://docs.unity3d.com/ScriptReference/Input.GetKeyDown.html