Juniper srx-100-b doesn’t boot anymore

Your config is possibly safe (if you care about them you should consider rancid or make manual backups) as it's on a different partition.

If you place the install image on a FAT32 formatted USB key you can reinstall JunOS from there, however you may need to use the reformat option which will wipe any config.

Juniper doc: http://www.trapezenetworks.com/techpubs/en_US/junos12.2/topics/task/installation/security-junos-os-boot-loader-usb-storage-device-srx-series-device-installing.html

You cannot decrease the maximum sessions value directly, but you can use Screen options to limit the maximum number of concurrent sessions per source- or destination IP. In your case I would expect a destination-based limit (see Juniper documentation).

For example:

security {
 screen {
  ids-option max-1000-sessions-per-host {
   limit-session {
    destination-ip-based 1000;
   }
  }
 }
 zones {
  security-zone untrust {
   screen max-1000-sessions-per-host;
  }
 }
}

However, it sounds really weird that your network is constrained by the number of sessions it can handle. This is usually only relevant for stateful devices like firewalls and load balancers, that are perfectly capable to drop exceeding traffic on their own. If it is bandwidth that's the problem, then there are probably better knobs to turn than session limits.