What should I do? A multisite, a single website or many websites? [closed]

You can have a different theme for each sub-site in multisite. Just set up the subsite, then select the theme from the available ones. If the sites are related to each other in some way, then having a common domain name might be an advantage (although there are some redirect things you can do to fix that).

Based on the limited information you provided, I’d go with a multisite. Add lots of available themes as needed to the main site, then select the theme you want to use for each sub-site.

Note that multisites will create 8-10 tables in your database for each sub-site, so make sure your hosting place will support that. Some hosting places will want you to use a premium plan for lots of tables in a database, rather than their base/cheapest plan.