[off topic complain]
I really don't like this ask simple question or tell ppl to search more... but ya it is really not the place to ask this type of thing... even ask stranger in the game could be faster. cus lots of us check forum every day and answer these 1 line answer... it's not that fun. but also feel bad if don't answer it...
I really want to suggest the community together to compile a "check here before you ask question" sticky post to reduce such one question thread. we can either link them to exist thread or Q&A there.

[on topic]
retainer will be 100 slots.
in the future, your equipped gears doesn't count as your 100 spaces.