You can transfer domains from one registrar to another or you can transfer domains between accounts within the same registrar. But not anytime you like. There are some time restrictions for consecutive domain name transfers.
When a domain is transferred from another registrar to your registrar account, there is a time restriction for you before you are allowed to transfer that domain again to another account whether it is with the same registrar or not.
A similar restriction is also applicable if a domain is transferred to your account within the same registrar. These are some security measures put in place by the authorities.
See ICANN policy for details: Policy on Transfer of Registrations between Registrars
When the transfer is internal, meaning domain is transferred between accounts within the same registrar, wait time can change between 5-10 days before you are allowed to make a new transfer. For example name.com says it is 5 days, godaddy.com says it is 10. So it changes from registrar to registrar.
If domain is transferred from another registrar, wait time can extend up to 60 days (contact your registrar for exact number of days).
Hope this helps.