I have a few questions about retainers.

First, Ive been crafting quite a bit, but I'm worried about running out of storage space. I get the impression that you can have more than 2 retainers, but I'm not sure how to unlock them. Is it level based or ?

Second, which classes are the most useful? I was thinking botanist and fisher for cooking mats, which would leave collecting the rest up to me. How useful are those classes for gathering usable ingredients?

Lastly, I'm a bit confused about the purpose of the adventuring retainer classes. What do these retainers do?

Thanks in advance!