How do you know how many 'end's you need?

They're a few things that require ends. If you have any of the following, you require an end

while / do
if / then
for / do
function

You should put an end where you want a 'scope' to end. It's difficult for me to describe a scope, so here's an example

while wait() do --start of our scope
print("Hi") --we want this in our scope still
print("This too") --also want this in our scope
end -- end of scope
print("We only want this to print once") --This will only print once, because it's outside of the scope of the while wait() do

With that said, you'll see things like

end)

This is because the start of the scope has an open parentheses somewhere. The most common place I see this is the following:

script.Parent.Touched:connect(function(hit) --Notice how we have two open parentheses, but only one close
end) --that final end parentheses closes the open parentheses, and marks the end of our function scope

I'll explain the odd looking function line if you message me on Roblox (my username is Lupine)

Hope this helped!

Lupine has described scopes for you, which is a good way of telling to add "end"s. ROBLOX updates, as hi12325 said, have added that some "end"s are automatically given. My advice is simple.

A red line will usually appear when an end is needed. Also when you run your script there will be an error saying something like "end expected somewhere" or something like that.

Refer back to Lupine's post for scopes, and remember that Studio usually helps you out.