There was a thread on AR15.com, discussing the minimum manning requirement for a site defense. I think it is useful to readers who are not paying members, to archive part of this thread. It also merits further clarifications on the tactical considerations for defense planning in a semi-rural/suburban area.
For a defensive position, you need to establish a perimeter, and station guards. Obviously, not everyone can be pulling security at the same time. You need to let people rest. Also, food production and other manufacturing activities are vital in a long-term situation. So only a small fraction of the group can stand security watch at a time.
The purpose of the sentry is to give time for the reserve/QRF to get ready. The time, in this case, is distance. Say your defenders need 1 minute to go from work detail to defensive positions. People can rush 200 meters (in gear) in a minute, give or take. So your sentry(ies) need to see at least 200 meters out from your perimeter. You may need 8 ppl per shift to cover that distance, but most likely 2 people, in a protected position (rooftop nest), will suffice. So 10 people are capable of handling that duty. I would go with 12am-12pm 12-hr shifts personally, as opposed to the 6am-6pm shift beloved by everybody else. It's easier on your biological clock.
[In a homestead, your perimeter is not the whole farm, but rather the living area complex (barn, house, outhouse(?), et al.)]
Where the people requirement really come in is in the active defense plan. In other words, patrolling. You have to keep up a patrol schedule if hostiles are in the area. The manpower requirement varies, but SOFs give us a guideline here. SAS go w/ 4-men teams, while Force Recon go w/ 6-men teams. If you have 1 team out, 2 teams resting/working, you need a minimum of 12 people. Factoring in the sentries would give you 4 teams min, 16 people.
Your sentry acts as your Observation Post(s). You can go up, or go out horizontally. If you have the altitude, a guard tower is good. If you cannot go up, then you have to put your sentries outside your perimeter. Speaking of guard towers, it is unconscionable to place people in unarmored guard towers. All guard towers need to be sandbagged at a minimum. Overhead shade obviously. A protected entrance. A rooftop nest with internal access is the best. Obviously the house need strategic sandbagging as well. Sandbags are easily accessible to the homesteader. An alternative is using those Amazon shipping box, with a plastic liner. Or just build a sandbox with plywood.
If you are in a built-up area, you will need more people because you need to have eyes-on for all of the dead space, possible infiltration routes. In addition, you need to subdivide your compound, so that infiltrators cannot compromise the defense of your whole site. In other words, you need to have "water-tight compartments", to use a naval analogy. For the perimeter defense, if you have a square, four firing ports/bunkers/strongpoints can complete your defense by putting out a wall of lead along the perimeter.
"Think of the effects of a probe on your BOL site not once or twice a day , but 10,12,or even 20 times a day and night."
Yes, it would be unwise to go battle stations with just a pot shot. Manning the defensive line/battle positions is only for an active assault, ie, you see people breaking from the tree line. And yes, you should clear as many trees as you can, out to 500m from your squad nests/houses, when possible. In a suburb/city, you may have to use obstacles to channel the enemy main assault, and rely on vigilance against the infiltration attempts.
"Lets revisit that probing issue. You say you will just send out a patrol to run them off. Thats a great way to get ambushed, ect."
Well, you do have to clear the area. Active patrolling means you should have a team in the field at all times. If you're doing it right, this is when you call the patrol team back to clear that sniper.
The preper needs to remember that the patrol team is out there to disrupt enemy assault preparations and recon operations. Therefore, the team needs to look for possible enemy objective rally points/patrol bases. They recon enemy ingress routes and vehicle staging areas. They check for signs of passage and occupation at these sites.
Speaking of vehicles, that will be the biggest threat to the survivalists. Think of it as modern cavalry. A pick up can greatly disrupt your defense, if you're not planned for it. You will need vehicle barriers [trenches and those giant caltrops/dragon's teeth] to block and channel a mechanized assault. You may also need to place an OP on the possible vehicle rally points for early warning.
The enemy mode of operation will probably be distributed, 1-vehicle scouting parties, ranging all over their area of ops looking for prey. Upon target selection, the scouts/swarm is recalled to mass against the target. Obviously the mass is depending on the perceived target difficulty. If you happen to be in the AO of such a gang, and you have just repelled the scout, it is time to run away. Unless you are organized and trained enough, or can call in enough posse, to ambush/interdict the gang coming your way.