Do you really mean "self-sketching pen", or can a hand be attached to the pen, as in the example video? There's a big difference.
Have you downloaded the example video to scrutinize it frame-by frame to see just what they're doing?
Do you have at leaswt an intermediate-level knowledge of both Illustrator and After Effects?
1) hand attached to pen
3)after effects yes , illustrator -begginer
I strongly recommend looking VERY closely at that video; I think you'll find that by looking at it frame-by-frame, you can see a bunch of shortcuts they took that you can also take advantage of.
If you're not that strong in Illustrator, I assume you'll be using an image that's already drawn... in which case, you may be in for some tricky masking rather than how you initially proposed to pull off this look.... a process that could be made easier by scrutinizing the example video
If somebody wanted me to recreate that video I'd bill them for 10, maybe 20 hours. You said "but when there are lot of masks it is a pain in the neck !!" That's right.... Animation takes time and what you are trying to do requires a bunch of time.
As far as copying and pasting the paths from Illustrator to a mask path in AE, that's easy, and if you select your paths in order and set up the stroke to stroke sequentially then you'll have your reveal.
Now, start with mask 1, set a keyframe, copy the keyframe.
Set the anchor point of your hand/pen layer to the tip of the pen. If the pen is moving (like a movie) then you'll have to track the pen tip.
Now set a single keyframe at the same point in the timeline where the lines are starting to be revealed by the animated stroke and paste. You'll get the motion path for the first path.
Now move to the end of the first line reveal and grab the last keyframe of the motion path and drag it to the CTI. move forward 1 keyframe, set a new position keyframe, select mask 2, set a single keyframe and copy, go back to your new position keyframe and paste. Adjust again and repeat for the 3 or 400 mask paths you have in the drawing...
Like I said, 10 or 20 hours even with cheats.
BTW, in illustrator, if your paths are all open, you can join the end points of paths to make a longer one. If the paths are closed there is no such option.
Like Dave said, there are lots of cheats in this video. The hand only kind of follows the revealed drawing. I'm sticking with my 10 to 20 hour estimate unless I can see the drawing you're trying to animate.
As Rick said, aniamtion takes time. just get used to it. There's not a magic button for everything.