error: no more sessions

sshd_config:

MaxSessions 20

# global
Host *
  User root
  ForwardAgent yes
  ControlMaster auto
  ControlPath ~/.ssh/control-%r@%h:%p
  ControlPersist 1h

In the default setup the basic escape character is ~ (you can specify this by using EscapeChar in your configuration)

<ENTER>~C

~.      Disconnect.
~^Z     Background ssh.
~#      List forwarded connections.
~&      Background ssh at logout when waiting for forwarded connection / X11 sessions to terminate.
~?      Display a list of escape characters.
~B      Send a BREAK to the remote system (only useful if the peer supports it).
~C      Open command line.  
    Currently this allows the addition of port forwardings using the -L, -R and -D options (see above).
    It also allows the cancellation of existing port-forwardings with -KL[bind_address:]port for local, -KR[bind_address:] 
     port for remote and -KD[bind_address:]port for dynamic port-forwardings.  
    !command allows the user to execute a local command if the PermitLocalCommand option is enabled in ssh_config(5).
    Basic help is available, using the -h option.
~R      Request rekeying of the connection (only useful if the peer supports it).
~V      Decrease the verbosity (LogLevel) when errors are being written to stderr.
~v      Increase the verbosity (LogLevel) when errors are being written to stderr.