First of all, ASUS B250 Mining Expert MOB doesn't support 19 normal GPUs. You need a mix of AMD and Nvidia cards to get all 19 GPUs to work together.
Ex. 13 AMD RX 580 and 6 Nvidia P106-100.
I'm not sure about the limit of Nvidia cards that you can plug but it might be 12 or 13 GPUs.
So, in practice, the board will only support 12/13 RTX 2080ti.
1. You can but I won't recommend it at all. For this, you can either get an
Add2PSU or
ATX 24 Pin dual cable.
2. As you are using RTX 2080ti, I'll recommend
6-pin PCIe Risers. I cannot stress this enough but do not use SATA cable to power riser.3. You simply cannot use a Celeron processor as it doesn't have as much raw power to handle this kind of configuration.
So, it must be i5 or i7 and minimum 8GB of RAM.
Also, do not use Windows, go with any Linux distros you feel comfortable with or mining oriented Linux distros like Simplemining, HiveOS, etc.