I don't have a number for you.
I'd like to point out that they have their origins in
HH oaths of moment, or vows to achieve a kill count or task.
They perform the following functions when bestowed by a chaplain/priest:
-Proclaim purity of the bearer
-Protect spirit of the bearer for corruption
-record vow or task to achieve a goal
In addition, a techpriest/techmarine might confer one:
-Proclaim purity of the equipment (checks/maintenance/blessing performed- maintenance log/update notes)
-Protect spirit of the machine from corruption
-record deeds
You potentially could get one from your chaplain to guard your soul, another to task you with deeds and one from your armourer for each piece of equipment (armour, firearm, sidarm, melee weapon, back banner, servoskull spotter)
And then a further seal like a crusader seal that might just contain holy writ or catechisms of hate...
Aesthetically I reckon that seals get grouped by function. The ones on the left leg probably relate to similar things- killing a number of the enemy, kiling an enemy champion, casting down their idols.
On the right, you might have blessings against the speech of the enemy, the horror of the enemy form, blessings of hate.
On the crozius, blessing log, specific blessing to sunder an enemy weapon.
/conjecture
The chaplain's time is limited- there are 100-ish marines in the company to bless. I expect that's the limiting factor on seals, which is why veterans get more- they get more clout and they have more deeds to record.