From the models listed you should be able to get 8 blood counters in one turn.
Each killed merc can generate 1 blood from blood sense, 1 blood from drain blood, and drops 1 corpse token which can be turned into blood via the shamans spell trigger. Add in the 2 blood from blood offering via nekima and the shaman and you get a total of 8 blood.
Unfortunately the order that you have to do this in means that you wont be able to heal the shaman. Nekima has to activate before the shaman so that she can add the mask to the cast for the shamans trigger. The two mercs have to be killed before the shaman activates so that he has corpses to turn into blood. When the mercs die they heal nekima since the shaman hasnt used blood offering yet. So when the shaman finaly activates he can use blood offering and inject blood on whatever to turn the corpses into blood, but he cant target himself with inject blood. Inject blood is a melee attack spell and you cant target yourself with a melee attack. So you cant heal your shaman back up if you blood offering with him unless you forgoe turning the corpse counters into blood because you have to use the mercs to heal him.
So you can have 7 blood and fully healthy models or 8 blood and a shaman with 4 wounds left all on turn one. Alternatively you can wait till the top of turn 2 to turn the two corpses into blood and get 9 blood and a healthy shaman.
The order of activation would be something like this:
1) Activate Nekima use blood offering and the +mask to casting values (1 blood)
2) Activate the Shaman use blood offering and inject blood with trigger to heal nekima and keep your bloods. (2 blood)
3) Lilith activates Kills a merc and uses drain blood. Blood sense from nekima or shaman generates a blood. Mercs death heals shaman. (4 blood, 1 corpse)
4) Activate a tot standing next to the remaining merc and grow. The now Young tot attacks twice and kills remaining merc easily then uses drain blood. Blood sense triggers and the mercs death finishes healing the shaman. (5 blood, 2 corpse, 1 young)
5) Tot activates grows then matures. (2 blood, 2 corpse, 1 young, 1 mature)
6)Last tot activates and grows. (1 blood, 2 corpse, 2 young, 1 mature)
7) end of turn
8)Nekima activates uses blood offering and adds +mask to casting. (2 blood, 2 corpse, 2 young, 1 mature)
9)Shaman activates uses corpses to cast inject blood twice with trigger healing nekima and turning two corpse counters into 2 bloods. (4 blood, 2 young, 1 mature)
10) Young activates and matures (2 blood, 1 young, 2 mature)
At this point we have the tots all up sized as much as possible and enough blood to mature the young once one of the matures dies. Of course this does eat up a good bit of your 2nd turn. So depending on the schemes this is not the way to go. This is however the optimal usage of the two mercs if you only consider blood generation tot growth and model health. When other considerations like grabbing a treasure come into play your probably better off trying to do as best you can in 1 turn. Afterall we do have 2 xtra blood at the end of this that we have no imediate use for.