Welcome to the Aligners community.
To answer your question, all companies that offer clear aligner treatments will always provide top and bottom aligners whether this is for crowding, gapping or any other issues. This is because the mouth should be treated as a ‘whole’ not separate top/bottom teeth.
Yes - it’s typically 6 months + 2 weeks retainer. And then you only need to wear your retainer at night so that your teeth stay in place. There are treatments which could take a bit longer like SDC’s night time aligners (10 months) and sometimes if you need refinements after the initial 6 months, this can add on a little bit of time.
Let us know if you have any other questions!