ItsUncertainWho said:
Well, they are a very specialized group, much like Deathwatch or the Grey Knights. They're not meant to be thrown into any battle - that is the purpose of the PDF and the Imperial Guard. If more elite forces are required you send in Stormtroopers or (as the final solution) call the Space Marines. Sisters Militant are only called into action when sacred relics or important Ministorum officials are threatened, some Cardinal calls for a Crusade or an Inquisitor requests their assistance for a single mission. If these things do not happen they keep going on with their daily schedule of playing bodyguards, protecting pilgrim routes or important shrines, testing people for genetic mutation and policing the Ecclesiarchy.
Their influence does not come from being omnipresent (iirc the Calixian Sector Ecclesiarchy's main headquarters are guarded by a single squad of Battle Sisters), but from being able to pop up anywhere they are needed, and from having the support of the entire Adeptus Ministorum - including Frateris Militias and the local clergy - and, by extension, of the Imperial Guard/Navy and the Adeptus Arbites. In fact, they often team up with the latter for the aforementioned purity control tests. When it comes to purely political influence, however, it is likely true that the Orders Famulous are able to pull a lot more strings than the Orders Militant.
It should be noted that the 30k does not include the Battle Sisters of any of the smaller Ordos Minoris, though the total number should not be massively dissimilar.
When it comes to active mission strength, the Deathwatch core rulebook contains a list of Sororitas units attached to the Achilus Crusade. If I recall correctly it was about 2.000 Battle Sisters compared to 1.600 Marines.
(or a whole squad)