When you put the panels in series, the voltage adds up. So do the total watts. To find the current of the series line, divide the total watts by the total voltage. The 6548 manual has an example of how to do this on page 9.

In your case, the panels run at 33V, with the 38VOC. You can make sets of 6 in series, with 6X38=228V. Working voltage is 6X33 = 198, let's call it 200. The total actual watts of that line will be around 1200, since these panels run at 75-80% of their stated output. So 1200W / 200V = 6A. When you put the strings in parallel, the voltage stays the same across the strings (if you measured from the positive end to the negative end) and the current adds up.

Since the current limit on each PV input is 18A, you could do 3 parallel strings of 6 in series (6S3P) and be right at 18A on the entire solar array. In your case you will have a 6S2P system with 12 total panels and the current in the PV line will be 12A. The example in the manual shows one case of 6S2P on each of the PV inputs, for a total of 24 panels.

BTW I'm in the middle of doing a similar system. LV6548 and panels from San Tan Solar. I read through the manual yesterday to get familiar with the install process.