O servidor SSH pode impor esses limites com a opção MaxSessions
no arquivo de configuração. No entanto, de acordo com ssh man
MaxSessions
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.
MaxStartups
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
implementa o protocolo completo da versão 2 do SSH no cliente e no servidor. então, acredito que você deveria executar algo parecido com este comando:
ssh stream tcp nowait/3 root /usr/sbin/sshd sshd -i -4
Isso limita o número de conexões ssh simultâneas para 3. Ajuste como você gostaria.