Сервер Ubuntu, несколько одновременных удаленных пользователей

Вам необходимо увеличить разрешенное по умолчанию соединение с помощью файла

/etc/ssh/sshd_config

, добавить в этот файл строки

MaxSessions 50
MaxStartups 50:30:100

. Затем перезапустите систему.

Форма man sshd_config (5):

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).