Webmasters are advised not to apply too many Javascripts on their website, as it will make the websites heavy, thus affects their loading performance. I agree with it, but I wonder how many is too many.

Well really there isn't a set amount that a user can handle. It all comes down to the specs of the machine that they are running and what their internet connection can handle. Caution is advisable.

I would wonder how you can tell which things being used, well, what their weight actually is? Do programs/applications come with a weight limit or how ever else they would be described as?

I would just suggest making your code smaller by removing all unnecessary comments, spaces and extra code. Also, couldn't you use a JS packer? That would make your code weigh less, but I don't know if that would make it render faster over the internet.

All I can say is that you can disable javascripts through your browser if you want to. It can however disable some sites completely, such as Gmail so you will have to remember to turn it back on.