Hi,
what's the difference? I think there's a difference... I think:
Look at how many/much... <--- Consider the number/amount of... (Look = Consider)
Look how many/much... <--------- Look at the number/amount of... because it's amazing or surprising.
Yes, you're right. I see #2 as characteristic of informal speech. One might argue that the 'at' is simply omitted. You might like to consider other uses such as
He looked daggers at me.
He looked me in the eye.
Look what he did.
I was told the expression "What a lot of...!" is not used much, Possibly not, at least where I live, although there is nothing wrong with it. If I walked into a room and was surprised to see so many people, I might say something like
There's a lot of people here.
I wasn't expecting so many people.
What a load of people.
Wow, lots of people.
so I wonder if people usually say "Look how many/much...!" instead. You could say this, but it's just one more possibility to add to the list above.
Best wishes, Clive