Here are the Lin Cable thicknesses and corresponding numbers:
Very Thick 332- .77mm thread diameter
Thick 432- 0.63mm thread diameter
Medium, 532- 0.57mm thread diameter
Fine 632- 0.51mm thread diameter
Very Fine 832- 0.43mm thread diameter
If you're familiar with Crawford's Irish Linen thread, the 432 is about the thickness of the 3ply and the 532 for the 2 ply.
The Lin Cable is really dense though and I'd say you could go finer to the 532 and get the equivalent strength of the 3ply or the 4ply
PS- We currently carry 432 and 532 Lin Cable (corded linen).