There is no "standard" number of backlinks for a website, it all depends on many many factors including age of site, quality of links, quality of content, competition of keyword, etc.

If you suddenly stop backlinking to your page, yes, it can harm rankings. It will give your competition more opportunity to outrank you anyway.

dont stop until you reach 10,000,000,000 backlinks. Thats a good rule of thumb.

Always keep up seo work on your sites

You should never stop doing seo on your sites. If you do everything your self and get ranked on page one and start making money with your keywords then you can start outsourcing the seo but seo is not something that you just do enough of and then stop.

Yup, you should always be continually building backlinks to your sites, from a wide variety of different sources. If you stop your link building, you will soon be surpassed by your competitors.

Unfortunately you can't stop it anytime . You should always continue it but its not needed that you build hundreds of backlinks daily. But never stop it. And the enough number depends on the competition on your desired keywords.