I used 6 bar clamps with my build and that was the minimum I would recommend. I would have liked to have had a couple more and at Menards they have 24" bar clamps for $5.99 apiece if you have one near you.
I used PL Premium since it expands nicely (buy some cheap work gloves) and once I assembled the panels together, I would hit them with my finish nailer (just a couple of 2" finish nails in each panel) and then clamped it. It really went smoothly and then I just used drywall spackle to cover the nail holes. You have plenty of work time with PL Premium so take your time and get everything lined up nicely.