When to go paid for my newsletter?

You don't have to go paid as if it were the only way to monetize a newsletter. There is an absolutely perfect good reason to go paid and only paid. That is that the people who read are the people who are most willing to pay.

The thing with most newsletters is that there are vastly more people who would pay to advertise to your newsletter readers, then would actually pay for the newsletter itself.

And you don't even have to sell directly to brands/businesses to advertise. I would experiment with Affiliate or Referral links first. In my particular case when I ran a newsletter to CEO's in particular the person buying a service and the person reading were different. I didn't find this out until an affiliate link I shared actually got quite a lot of clicks and 0 sales attributed, and thus $0.

But with small startups (I assume are reading your newsletter on traction) the person reading will most likely be the person paying for a service to help their business.

I'd look for affilaite links first, and if you can't find them a few easy advertising options are going to be mentor sites like GrowthMentor, MentorCruise, MentorPass.

But I'd also scour through Gumroad looking for reasonably helpful products, like Steph Smith's "Doing Content Right" and reach out and ask for an affiliate link for your newsletter.

Even if you get no sales, you will know how many clicks your ads get and then you can use that data to share with potential advertisers.

But this might not be the path you want to go. You don't want to advertise other businesses, you don't want to do affiliates. If you do go paid, you'll still have to advertise your own paid newsletter.

So I'd also consider creating info-products first, before going paid (subscriptions) That way you can test if anyone is willing to pay. And you can experiment with prices.

Create a google doc book of interviews that can be made into a nice well designed pdf. Probably you can do it yourself to do it cheap or hire a designer for less than $500.
Start it out at $50 and the first ten buyers give a coupon code to for $40 off, then then next 10 get $30 off.. and by it's nature you'll see how much you should really charge. if people don't pay $30, then charge $25. if ppl don't pay $50, then charge $45. you'll see where the sales drop off. And you'll be able to advertise a one off payment.

And if nobody buys, you can use it as a lead magnet when people sign up.

Or heck, keep a google sheet of all qualitative data you can pull from your interviews. Sell access to that google sheet. Update it every week. It's an easy thing to sell access to without having to go paid and produce more editions.

And there is no minimum sub count you need to get to go try any of these.