The SSH server can impose such limits with the
MaxSessions option in the configuration file. However, according to ssh man
Specifies the maximum number of open shell, login or subsystem
(e.g. sftp) sessions permitted per network connection. Multiple
sessions may be established by clients that support connection
multiplexing. Setting MaxSessions to 1 will effectively disable
session multiplexing, whereas setting it to 0 will prevent all
shell, login and subsystem sessions while still permitting for-
warding. The default is 10.
Specifies the maximum number of **concurrent unauthenticated con-
nections to the SSH daemon.** Additional connections will be
dropped until authentication succeeds or the LoginGraceTime
expires for a connection. The default is 10:30:100.
Alternatively, random early drop can be enabled by specifying the
three colon separated values ``start:rate:full'' (e.g.
"10:30:60"). sshd(8) will refuse connection attempts with a
probability of ``rate/100'' (30%) if there are currently
``start'' (10) unauthenticated connections. The probability
increases linearly and all connection attempts are refused if the
number of unauthenticated connections reaches ``full'' (60).
Dropbear implements the complete SSH version 2 protocol in both the client and the server. so, I believe you should run something like this command:
ssh stream tcp nowait/3 root /usr/sbin/sshd sshd -i -4
That limits the number of concurrent ssh connections to 3. Adjust as you would like.