"What's the temperature outside?" would be a totally natural way to ask this. You can shorten this to just "What's the temperature?" if you think it's clear that you're talking about the temperature outside. You can also add "now" or "right now" if you want.
If you know that it's cold outside, it would also sound totally natural to ask "How cold is it outside?"; and likewise, if it's hot, you can ask "How hot is it outside?"
"How many degrees are now?" sounds ungrammatical. On the other hand, the question "How many degrees are there now?" would be grammatical—but it's still incorrect! After all, we don't say "There are 30 degrees outside"; we say "It's 30 degrees outside". So you can't ask about "how many degrees there are".
You could ask "How many degrees is it now?", and that wouldn't be incorrect—but it's just not something people say. If you ask the question that way, people will think that maybe you don't know the word "temperature".
- What's the temperature outside? – good
- How cold is it outside? – good if it's cold outside
- How hot is it outside? – good if it's hot outside
- How many degrees are now? – ungrammatical
- How many degrees are there now? – incorrect (but not ungrammatical)
- How many degrees is it now? – sounds strange (but not incorrect)