A question about CPU

Turbo boost goes automatically, the same as other Intel Core CPU series. The cheapest (the one I have) has 8 core (16 threads). If you plan to use Final Cut with 4K videos maybe you want to purchase more cores. Depends by your budget, of course! With full HD video, I never saw the CPU cores reaching the max power and the effects just take a few seconds to be applied. Before this, I had a 2011, 3.4 GHz Core i7 iMac. Compared to it, this is really faster, although I can't estimate precisely. To me Is looking at least twice as fast the old Mac.

First, Turbo Boost is part of the CPU and is applied automatically. It's not something you can change. You can check the relative speeds of various CPUs on Geekbench Mac Benchmarks - Geekbench Browser

It's not so simple in real life. As a rule, the more cores the better but there are exceptions. For one thing a program has to be optimized for multiple cores. There are apps that will run in one core no matter how many cores are available.

There are some apps showing the cpu frequency and many other details. For example iStat menus 6, you can try for free several days. You will see the frequency changing continuously. On my iMac pro 8 core I use Final Cut Pro X at the most. If you want to see all cpu cores at 100% you can try software for 3D rendering like Daz Studio Pro (it is free).

I had the same question for months. There will be a new iMac Pro within a year after the first one? I couldn't wait further just because the old iMac was failing. But the more you wait, the more you get the chance the wait will be longer. How many times I bought a new Mac and a new one was around the corner. It is not pleasant at all. When a new one is coming, as usual they give you more for the same price. Maybe starting with 10 core as entry level. Actually I was waiting new iMacs in June, maybe with the sound and cooling systems of iMac Pro, which are really awesome. But didn't happen. Many were expecting the 20th anniversary iMac, but nothing yet. I would wait till the next Apple Event, maybe in October another event will come (last year didn't happen). After that date, the product for the holiday season are on the table, and nothing will come before the next year. Usually.