Depending on the duct type (and static design) and installation, you have a capacity of about 1800 cfm for 4 ton system. That 10" feeds one outlet? Or is it branched out to other ducts? In other words, if you have a 10" feeding a 6" and 8" round duct, then you would only count the 10" and not the 6" and 8" as seperate ducts. If the 10" drops to one 8", then you would count the 8" only.

Thank You

the duct work is sheet metal

I didn't double count the lines. e.g. the 10" line does branch and and I only counted it as one 10".

I think all lines either branch or do not reduce if it is a straight run.

There are 17 registers

What would Install a 3.5 ton or 4 ton?

Thanks again

The only way to properly size a system to your home is to do a heat load. A good HVAC contractor can do that. Don't fall for the "rule of thumb" (so many sg ft per ton ideas), as windows, doors, insulation, infiltration, latent loads, and other loads are not all the same in each home.

I told the existing duct system will only support 3.5 ton to 4 ton. Existing duct system (the nine pipes above) will allow 1550 cfm of air flow. Using one ton for 425-450 cfm you get something between 3.5 and 4 ton.
Using 6" duct 100 cfm
8" duct 175 cfm
10" duct 325 cfm

Let say the heat loss calc shows 80,000 BTU/hour when temperature is 20 deg F. Thus if new construction I install a 5 ton system (60,000 BTU/HR) and then size the ductwork to carry it to each room.

But my ductwork area is fixed so you have to work backwards. Right?

That or install new duct sized to the larger system.