Ignoring regular units.
The ones that you can get their SR+ from Silver Recruit are considered the common ones, with their SR+, R and N ranks being obtainable from it. (Lilium, Elena, Lisette, Brigitte, Adelite, Alette and Neun). You can get R rank of Platinum uniques from Silver too but only their R ranks, which makes them the rarer bunch of Silver Recruit. Just check the unit table.
Ranking up requires you use 1~2 copies of the same rank unit and any other uniques of the same rank. Can't use regular soldiers with any uniques.
Promoting a R to a SR normally needs 2 more R copies of itself and any other R unique. Elina R+ requires 2 copies and 2 any other Rs probably because of the being the + version.