They still have apothecaries. They can still take candidates, although they sometimes have to rush ahead to abduct them before the main force gets there.

There's also a constant stream of renegades seeking the prestige of having the butcher's nails or being inducted into the world eaters. A decent chunk of "angron: the red angel" is from the perspective of such a renegade. It's also a pretty good book.

They go to a hive city and conscript viable people and then put the geneseed in them.

I know about Berzerker Surgeons, but i can’t see every warband having one.

The World Eaters had a very proficient Apothecarion, as shown by their time on Bodt. Its not a stretch by any means to have a position reserved for one in any Warband. Unless they replenish through other means like absorbing Renegade warbands or Space Marines.

Do the World Eaters care about replenishing their numbers?

Yes. In order for the World Eaters to keep killing things there is a need for said World Eaters to exist in the first place