When is a startup idea considered successfully validated?

Based on what metrics would you consider a startup idea to be validated?

Number of people willing you give you money in advance? What if you aren't validating the idea by charging people (maybe you have a landing page to capture emails)? How do you do it in that case... 1K signups, 10K signups?

There is no set formula - it depends.

Getting email addresses tells you that people are prepared to hear more about your idea. Nothing more. By handing over their email address they have taken no effort or risk - so your idea isn't validated more than "it might be interesting to X many people". In my opinion, this is fast heading towards a redundant mechanism.

Getting payments, in advance of your product/service/thing existing, is a sign that people want what you're offering badly enough that they'll risk their money in advance - but again, the amount paid, influences how risky their position is.

What works best for you, has a lot to do with what you're offering. If the item is low cost, low risk to you, then fewer signups/dollars is an acceptable validation. If the cost (money, time, technology, materials, etc) is large, then you want a more certain understanding of validation before making commitments.

I recently saw someone suggest that they would be interested in writing a book about a certain subject - they created an ebook,empty except for a contents list, and said that once 5000 people had signed up for it, he would write the content. Granted, there was no financial pain, but you did have to go through some effort to sign up (as opposed to just adding your email address). The content is currently being written and published section by section to those who signed up.