Two parts: what the motherboard can support and what the RAM modules can run at.
The guiding document is the Motherboard's User Guide/Manual.
https://www.manualslib.com/products/Asrock-B450-Steel-Legend-10771269.html
Do verify that I found the applicable manual. Make, model, and revision matter.
Supported motherboard RAM configuration are listed starting on physically numbered Page 22.
How most manuals also refer you to the Motherboard manufacturer's website for more and updated information.
You can also find similar information via the RAM module manufacturer's website. Look for a consensus.
Details matter so read all the footnotes and fine print.
Do not exceed the specs and it is recommended that you start at the low end and test upwards in increments.
In any case ensure that the PSU is able to support the overall system power requirements.