My team is searching for headless CMS with the possibility of supporting multi tenant architecture. CMS should be free and shouldn't be based cloud or something like that. Also CMS should work with keycloak.
Webiny is and open source headless CMS. It has multi-tenancy built and can be configured to work with any auth provider. The default is AWS Cognito but you can write a plugin to use KeyCloak as long as it has a decent API.
Not sure what you mean by "cloud based" though ... Webiny is a serverless CMS, so you host it on your own AWS account. This gives you a huge range of benefits as well as cost-effective hosting.
Hope that helps.