Do spaces in your URL (%20) have a negative impact on SEO?

Yes don't use them - Google, Yahoo and bing does not know how to leverage the spaces and more importantly you are wasting good opportunity to communicate both with the consumer and search engines more about your product or page URL and what the topic of the content is all about.

However, sometimes it can't be helped because you have a website / ecommerce site for years and the site is indexed and already on good page ranking.

In that case, if you do want to get better naming convention, you will want to re-name the urls but take all of the existing url with space and place it into 301 redirect and map them to the new urls.

I personally think it should be "-"

I don't remember seeing a website that was using %20

"-" is one character and %20 is three, so you can put more stuff visible in the address bar

for an example, what is better

Do spaces in your URL (%20) have a negative impact on SEO?

or

Do spaces in your URL (%20) have a negative impact on SEO?

No impact on SEO. A - just looks nicer, that's all.

You'd use %20 if you needed to preserve the exact term including a proper space when you read it back from the URL. Probably you don't.