When I first started toner transfer, 15/15 mil was pretty much fine. Any larger didn't really help that much.
I can comfortably do 8/8, now, pretty much all day. And the main limitation is the resolution of my printer. But if you want to get that fine, you have to pay attention to more details in the process. There are several ways to improve toner transfer, but you have not specified, so I will not dwell on toner transfer in particular.
In general:
1.Area of the board matters. If you want to run traces through DIP pads, this should be no problem. Just make your DIP pin pads rectangular, so you can make them thinner. If they break on the side of the hole, no problem. Also, you can make the traces thinner locally, them fatten them up again. The larger area you try to cover in smaller pitch/spacing, the more likely you will have a failure. Esp if your board is very small, feel free to push the boundaries. Just make several more than you need to account for any failures.
2. Thickness of the copper layer: The thinner the copper, the more accurate your result. I would advise you get copper clad in 1/2 oz, 1 oz, and perhaps some single sided 2 oz boards for DIP PSU/power stuff. For tight pitch, use the thinnest copper you can.
3. Etchant: etchant matters, particularly when the copper is thicker than 1oz. HCl and peroxide is perfectly capable of making good boards in 1 oz pour, but it becomes difficult at 2 oz. Different etchants have various degrees of undercutting, and HCl + peroxide is far and away at the bottom of the barrel. It is the most effective at removing the resist before the traces are done etching.
4. "Shading" When you get to traces smaller than 12, you may want to start paying attention to routing. Esp in an etch tank with any turbulence/agitation/flow, a lone trace with nothing around it will tend to etch more than traces that run together in a pack. When doing fine pitches, I will preferentially use a ground pour to fill in blank space to reduce this sort of etching problem. Keeping consistent space between all areas of copper increases the probability that all etching finishes closer to the same time.