There is no way to say that. It could be 100 per day or even 1000's.

there is no answer to this without knowing what you have done

Not only is there no way to answer this, not knowing what methods you have used to optimize your site, get backlinks and market your site, but there is no way to know this without knowing what your website is and what keywords you are targeting and niche the website is in.

I guarantee you that a website that has done a reasonably good job at SEO and marketing that is in the consumer electronics industry has WAAAAAY more traffic than a website that has done the same SEO/marketing job but is in the "how to raise baby elephants" niche.

Yeah, this would be extremely hard to tell as a lawn care company in the middle of Nevada with a "basic SEO-ed" website, would be different than an e-commerce website. Also the competitiveness of the terms you did SEO for will matter as well. Looking at traffic year-over-year for your website to see if there has been improvement or not is a better indicator than looking at industry website averages.

That depends on a lot of different factors, including your targeted keywords' search volume and their rankings in Google. If you target keywords with a high search volume and are able to rank those keywords on the first page of Google, then your site could possibly receive hundreds to thousands of visitors per day. However, if you target keywords that have a low search volume, then your site can receive 10-20 visitors per day (for example) even if ranking at #1.

How old your site it is not important. If content is lame, you will get no visitors.
But if you hit trend topic with your site, you will be pretty successful with it.

I'm not doing SEO anymore for the past 1 years and my sites have a lot of visitors per day.

If you can answer the average number of traffic with no SEO method I can answer you back. lol~

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No one can answer this. Personally, I've reserved a 3-month-old Website with able to get 2000+ organic visitors per day. It's because of the Website able to rank on the first page of google with a High Search volume keyword. How many visitors you have depended on your keyword selection and ranking.

Somewhere between 0 and more than 0.