Is bios update required for ASRock B450M. Ryzen 7 3700x.?

Any socket AM4 motherboard that you can buy at newegg or amazon is going to work with the R7 3700x because the 3700X has been out too long. The only people that had to worry about updating the BIOS were the early adopters. 

You should get a b550 motherboard, anyway. There isn't much of a guarantee that a b450 motherboard will work with the new Ryzen 7 5800X, and they won't work with the new Ryzen processors right away. 

You can google this. As far as I remember no you will need a bios update. Depending on the manufacturing date.

Probably not, however it is best to have the latest!

the AsRock website, under that model, has a list of all cpus it has been tested with and found to work.  if a BIOS/UEFI update is required, it will say so  [you'll find this under 'support' and then 'cpu' for that mobo model]

BIOS Update.....?

Possibly but not likely. 

This would apply to b450 and x470 motherboards that were made before the 3700x was released. Anything that's been made within the last year will have the updated BIOS. Normally a motherboard was manufactured 2 months before you buy it so it should be fine. 

The only way it wouldn't work is if the motherboard had been sitting on the shelf since July of 2019, which is possible but not likely.