If you're interested in this topic, Anthropic Principle, is a good place to start.

It's very challenging to say. People have tried to ballpark it, concepts like Drake's Equation. But they all rely on assumptions that we're really not capable of asserting.

Still, it does allow us to consider the topic in ways that are more fruitful than just random consideration. After all, as technology progresses, we do become better at defining many of the assumptions, giving us a more accurate range in which to find intelligent life.

I think most considered people accept that life in the Universe is robust. Intelligent life however?

That's a much harder thing to say, because it requires a lot of things to go just right. Many of which (Abiogenesis) we don't even understand.

So we search. But that poses its own unique set of challenges. The issues of searching such a vast area for signs of intelligent life.

A lot of really smart work has gone into narrowing bands of information that we'd expect to see from intelligent life. From particular radio frequencies to different combinations of chemistry that would be challenging to reproduce naturally.

But still. The Universe is beyond massive. Needle in a haystack isn't the appropriate comparison. Needle in a haystack of a sky full of haystacks is the more apt one.