CiviSites Basic is for people who want to run a small instance of CiviCRM for managing their contacts. CiviMail is not enabled and neither is CiviCase but everything else is there. There is no actual limit on the number of contacts or users, but larger instances are more likely to want additional features and support.
If you just want to experiment with CiviCRM or CiviSites for while this is an ideal starting point and you can upgrade at any time to another plan.
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