Isolate and suspend PBMCs in completed RPMI (containing 10% FBS, 1% penicillin-streptomycin). Then, mix 5 ml from this cell suspension + 4 ml FBS + 1 ml DMSO and quickly aliquot them into cryovials for gradual freezing in vapor phase liquid nitrogen.
Freezing 5x10e6 to 10e7 cells / ml / vial is fine.
It may be possible to cryopreserve cells for future cytometric analysis. The isolation of PBMCs takes advantage of differences in cell density of the different blood components. Density gradient centrifugation of diluted whole blood layered over a density gradient medium yields PBMCs; two subsequent washing steps remove remaining platelets. To store the cells for future assays, they can be frozen and thawed when required. Dimethyl sulfoxide (DMSO) serves as a cryoprotectant for freezing PBMCs, but must be removed by washing after thawing, as it can become toxic to the cells on longer exposure( Riedhammer et al.,2016).
You can store the blood sample at room temperature for a day and still get a reasonable yield of PBMCs. That is probably the longest you could preserve it before PBMC yield/quality is impacted.
The best thing you could do is isolate PBMCs from fresh blood samples using density gradient centrifugation immediately after blood collection. Several commercial isolation media (Lymphoprep, Ficoll) and specialized tubes (Sepmate, Leukosep) exist that can facilitate isolation. Once you have isolated the PBMCs, wash them and freeze them in an appropriate freezing medium (FBS-10% DMSO, CryoStor-CS10) and store the vials in the vapor phase of a liquid N2 tank. You could freeze anywhere from 5-25 million PBMCs/vial.