Event.clickCount
var clickCount: int;
int clickCount;
clickCount as int
Description

How many consecutive mouse clicks have we received.

Used in EventType.MouseDown event; use this to differentiate between a single and double clicks.
	function OnGUI() {
		var e : Event = Event.current;
		if (e.isMouse) {
			Debug.Log("Mouse clicks: " + e.clickCount);
		}
	}
using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void OnGUI() {
        Event e = Event.current;
        if (e.isMouse)
            Debug.Log("Mouse clicks: " + e.clickCount);
        
    }
}
import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def OnGUI() as void:
		e as Event = Event.current
		if e.isMouse:
			Debug.Log(('Mouse clicks: ' + e.clickCount))