It really depends on the selected hardware and how easy/cheap it is to produce said hardware. There's a reason the Switch and Series S are more readily available and it's not because people do not want either of those, but because they're easier, cheaper, and quicker to produce the parts for, so they do not suffer the same supply issues the PS5 and Series X are dealing with.
Maybe about 1 or 2 million in the first year. Then it will explode in sales if Valve delivers. This could be Valves "Sony Moment" when Sony entered the console market with PS1. If thare are minimum faults with the device then it will be a huge success.