How many 5 inch ducts for a 16 inch trunk line?

[Sigarms]

I plan to tape the plastic duct to the first section but do not plan to tape the rest as a little air leak would not hurt, I want to keep it neat and clean looking. I could use silver tape if needed.

Seal ALL your joints (edit, after thinking about it after a walk, duct work will be in a conditioned space, guess it's not that important, just that it's good practice to get into).

If possible, after the unit is running, check static pressure and air flow and compare to the blower table in the install manual to verify proper operation.

The other week had to replace a second compressor in the same amount of years on a package unit. Tech informs me that the ductwork looks fine, can't be the issue. Have him run some tests. Turns out the original installer lowered the fan speed to get an acceptable static pressure (existing duct work, crawl space), the only issue was he wasn't getting enough CFM's by lowering the fan speed to get the static pressure he was looking for. Being that you have a new "open" duct system, shouldn't be a issue, but it never hurts to verify it's operating properly.