I don't think we have any idea of how many navigators are active at a given time.
On the general note, I suspect that the navigators have found a way to create new compasses.
My opinion comes from several details :
- there's a lot of guilds, but for some reason they all work as a corporation.
Maybe this reason is that it's the best way to keep the secret of
compass creation a secret.
- It is stated somewhere ( I know, it's heretical : Stormriders ) that once atuned a compass is extremely difficult to use by somebody else. Iarsang explain a lot about compass by what he says and what he doesn't say.
So we can more or less suspect that, for these reasons, the Guilds don't bother with getting back the compasses of the deads. That means they have to have a mean to create them.... or a huge stockpile of them.
- I don't know where it is found, but I remember seeing a short 'flavour text' that described the discovery of some of the first compasses...
From what I gather of this discovery and of the various shapes and guilds, there's been several discoveries of several compasses... but no way there's been enough compasses for all the navigators, so they have to have found a way to create them.
- Last, like everything else Kulthean Andraax has probably overseen and driven ( from the back, and hidden ), being who he is, he has probably 'given away' the knowledge of compasses creation ( through devious means, like putting the searchers in the right direction and nudging them back on track if they wandered off ).