how many threads Photoshop can use at most?

A fast and efficient disk configuration (NVMe, PCIe 4.0) has more direct impact on operational speed than a fast CPU.

As for CPU, an i7 is the sweet spot. Above 8 physical cores you'll have rapidly decreasing return on investment. It won't be worth the cost - although it might pay off for a more CPU-intensive application like Lightroom Classic.

In any case, more and more tasks are performed in the GPU these days. Again, you don't need anything high end, but you do need a current and capable GPU.  Again, there's a sweet spot: Nvidia RTX 4060 or 4070 (or the previous 3000-series).