What's the finest grit stone I will need?

I'm looking around at different water stones, ands wondering how fine of a grit I need to get a razor shave ready, regardless of how polished the bevel looks. 4k? 6k? 8? I've heard it can even be done off a1k with slurry. Thoughts?

I think the generally accpted low limit is 8k. I found a 5k to be acceptable. Lower than that and I would say either its a matter of what you can live with in terms of comfort, but why bother with a atraight?

I have shaved off a 1k, but I won't do it again. I wouldn't shave below an 8k again and prefer anything above a 12k edge for my daily blades.
So, while it is possible it isn't worth it to save some bucks. Get yourself a norton 4/8, you won't dislike it.

As said above, 8K. In terms of natural stones, I have shaved comfortably off a Belgian Blue, which many regard to be the equivalent of 4-6K. Much smoother though, to an artificial comparable, being a natural. Go for the Norton 4/8K or Naniwa 3/8K. Both can be used to bevel set and make a razor shave ready.

I have a 1k/6k stone on the way, and will most likely buy a norton 4/8 late and maybe a12k down the line.

i use a 1k/6k followed by a pasted strop.... and i have a chinese 12k on the way ...... the 4k/8k seems to be the way to go.... if i were you that is what i would do... norton 4k/8k....

My finest stone is a 5k Naniwa, but I finish on .25 diamond spray on hard wool felt after that. The edge is very smooth, much like a coti.

Do you really mean need or want? It's kind of a loaded question because like the others have said you can get a very good shaving edge off an 8K maybe lower but you will most likely aspire to a higher degree of comfort and desire higher grit hones at some point down the road sooner or later.

An edge is 98% there after your 4k stone in terms of sharpness and fully there after the 8k. With the right touch you can get a huge amount of smoothness out of the 8k as well.

After that it's really down to what you want to spend and what finishers you like.

Remember, high grit doesn't necessarily equal smoothness. An Escher is probably about 14k (guesstimate) but is smooth as a smooth thing. 0.5 Diamond spray is about 30k grit but not as smooth as an Escher IMHO.

I'm at the other end I think, I've got a CF (no idea what grit they are but quite high if memory serves) so need something lower to get the edge. Also got a Silkstone but that needs serious lapping (again, no idea on grit). Have access to a nice India oilstone but they're quite low from memory - though gives a nice slurry and good for setting a bevel IIRC.

As others have mentioned...nortons 4k/8k. 8k may be sufficient. The nortons are easy to use. For the money, the naniwas 12k is great and has worked well with the nortons. The naturals get pricey. I started with the norton 220/1k, 4k/8k, and a bit later the naniwa 12k before getting into naturals. I think it was the right path. I wouldn't have done it any differently.

Until you have learned enough that you can get regular good shaves off razor you have freshly honed to 8K, followed with ONLY leather, you don't need anything else.

Finishers (any stone considered above 8K) are interesting, expensive, and I really think "faddish". What I mean by that is the highlight will be on one stone or another. It is fading on the Escher/thuringians and starting to lean towards the Japanese natural finishers. I have a handful of finishers that go up to 30K. If I were to recommend a "first" finisher for someone it be a Naniwa 12K.

It actually appears as though you are just starting to entertain obtaining your first stones & that recommendation would be a Naniwa 3/8K or Norton 4/8K. Stick with one mfg if you are looking to get a full "system" from 1K on up... some stones don't mix and match well.