So you want to be a Engineer Player?

It is not only level of your Loaded Engineer skill which dictate how many constructions you can build, but also your character level.

so you can build:

1 sentinel at level 30
2 sentinels at level 70
3 sentinels at level 120
4 sentinels at level 150

1 vehicle at level 20
2 vehicles at level 40

1 turret at level 30
2 turrets at level 70
3 turrets at level 100
4 turrets at level 150
5 turrets at level 200

Source:

[DruidsRPG227.EngineerPointsInv]
SentinelAvailability=(Number=1,Level=15) You actualy need Player Level 30 (5*6) to get LoadedEngineer Lvl5, which allow you to get Summon DefSent artifact
SentinelAvailability=(Number=2,Level=70)
SentinelAvailability=(Number=3,Level=120)
SentinelAvailability=(Number=4,Level=150)
VehicleAvailability=(Number=1,Level=20)
VehicleAvailability=(Number=2,Level=40)
VehicleAvailability=(Number=1,Level=60) These values seems to be omitted since level 20 already allows 1 vehicle and level 40 allows two vehicles
VehicleAvailability=(Number=1,Level=80) ...
VehicleAvailability=(Number=2,Level=100) ...
VehicleAvailability=(Number=2,Level=120) ...
VehicleAvailability=(Number=2,Level=140) ...
VehicleAvailability=(Number=2,Level=160) But I am no expert and I might be wrong.
TurretAvailability=(Number=1,Level=30)
TurretAvailability=(Number=2,Level=50)
TurretAvailability=(Number=3,Level=100)
TurretAvailability=(Number=4,Level=150)
TurretAvailability=(Number=5,Level=200)
BuildingAvailability=(Number=1,Level=4)
BuildingAvailability=(Number=2,Level=10)
BuildingAvailability=(Number=3,Level=15)
BuildingAvailability=(Number=4,Level=20)
BuildingAvailability=(Number=5,Level=25)
BuildingAvailability=(Number=6,Level=30)
BuildingAvailability=(Number=7,Level=35)
BuildingAvailability=(Number=8,Level=40)
BuildingAvailability=(Number=9,Level=45)
BuildingAvailability=(Number=10,Level=50)
BuildingAvailability=(Number=11,Level=55)
BuildingAvailability=(Number=12,Level=60)
BuildingAvailability=(Number=13,Level=70)
BuildingAvailability=(Number=14,Level=80)
BuildingAvailability=(Number=15,Level=90)
BuildingAvailability=(Number=16,Level=100)
BuildingAvailability=(Number=17,Level=110)
BuildingAvailability=(Number=18,Level=120)
BuildingAvailabilit)=(Number=19,Level=130)
BuildingAvailability=(Number=20,Level=140)

LinkTurret can boost damage and get you xp when you link other players equiped with LinkGun or EngineerLinkGun, but it is not working when you link another turret.

I hope it will get fixed/changed someday.

EDIT2: update2

hmmmm my eng is lvl 130 and can only do 2 sentinels

You also need to have enough of construction points.
If you bought all fifteen levels of Loaded Engineer, then you should be able to summon three defensive sentinels

I'm about level 217 sentinel master. I can summon 4 defensive, one offensive and two defensive, or 2 offensive sentinels.

I hope that goes up......

i love engineer class and i love medic class and genal class i so new at it.