10skinny10 :
I'm building gaming PC. Can I pair Ryzen 5 2600X with mATX motherboard ? Will it be stable ? Bear in mind that I have tight budget so I cannot really afford X470.
Yes you can and it will be stable. X470 is only essential if you have need for the PCIe lanes as you can get two full bandwidth PCIex16 slots and two full bandwidth NVME's only on X470 boards. To justify their high price they also get the best VRM's so some (the very most expensive) are the overclocking champions. But if those things aren't important then B450's are the best choice.
The best mATX boards are the MSI B450M Mortar and Asrock B450M Pro4. They have good, strong, cool running VRM's that enable even 8 core chips (2700x, 1800x) to run with boost performance comparable to X470 boards. They even overclock as well as the lower-end X470s. Your 2600x will be a great choice, with 6 cores the sweet spot for gaming.