It depends on how you use the boat.
I recommend having six (five minimum) on the boat all the time. We have that, but then we also have five lines in the slip that we leave there when we go sailing - L-shaped slip with the usual springs x2, bow and stern to main pier plus additional stern going to the other side. Obviously a little more cost having additional lines but makes docking
easier. The six on the boat live in the locker with little use so far (newish boat).
As for length: two at least as long as the boat (so about 15m min. for your boat, proposed 20m is good), two short (half length of long ones, so 10m) and two intermediate (15m). For ones that stay with the slip they could be customized to the application and potentially much shorter (to save a bit of dough).
If you mostly sail in and out of your slip (like we have so far) the boat lines can be more basic. The more you dock
in other locations it's nice to have more quantity and variety.
I didn't look up your boat specifics so won't comment on diameter. I know its a cat, but not weight.
edit: Harry got there first. And yeah, longer is usually better.