How many PCB layers?

One of the first decisions you need to do when starting a board design is number of layers you want to use. Many simple jobs can be done with just single layer, and for DIY milling/etching that might be the easiest way. However, since you can get professionally manufactured 2-layer boards for next to nothing (link as an example, there are others), I don't even bother myself anymore.

For many purposes 2 layers is sufficient so you might not need to go higher than that. However, there is a point where designing PCB with two layers is just too much of a trouble. Maybe there are too many traces, maybe ground and power planes (well, traces) get too broken, maybe you just want to pack lots of components on a small board, maybe you want to to that nice chip with fine-pitch BGA package with lots of balls.

Next option is 4-layer board with vias fully through the board. The good thing is that you can do a lot with this setup, and this is still easily available, even for prototyping (itead, linked above, takes about $10 for 2-layer 5x5cm board; 4 layers is $65. More expensive certainly, but not that much really).

At some point even that gets too difficult, especially if you have components on both sides of the board, so you may want to go for blind or buried vias. They're great - you can have a via from layer 1 to 2, and another via (with other signal) from layer 3 to 4, at exactly same location! Imagine how much more you can fit on a board with these!

Unfortunately they come with a cost, in this case literally. For prototyping there aren't that many PCB manufacturers who want to make them, and cost can easily jump to $200+ range (and that's the cheap end) for just a few boards. That might be a bit too much for you, especially since you may need several iterations (each costing that $200) to work out issues of design.

The good thing is, however, that if you are looking for even moderate production runs (let's say 100+ units), the actual production cost difference is tiny. If 2-layer board costs, say, $4, the 4-layer with blind vias might just be $8 (with initial costs included). And with larger runs the difference gets even smaller.

And even better, going to 6 or 8 layers just barely increases the production cost. Initial costs are even higher, yes, but per board the increase might actually be very small. So, unless you are planning to do only a few boards, there really isn't any reason not to go for blind vias if they make your life easier.

So what to do if you eventually want a 4-layer board but don't want to waste too much money prototyping? One option is to make larger board initially: Say you want your final board to be 5x10 cm. You first design a two-layer board with size of 10x20cm. No need to even have all the traces there - if some signal is too difficult to route you just replace it with pads at each end and connect it with a wire. That's good enough for testing the initial design, as long as you pay attention to high-current and high-frequency traces between board parts.

Then, when you design works well enough, you can scratch the prototype PCB design and start over with different board structure, moving components to their final (or close to final) posititions, change board type and start routing it again. Yes, this adds lots of work, and must be done very carefully to avoid routing mistakes when essentially re-doing entire board, but if your monetary budget is tight but work time cheap-ish (as it often is with hobby-type projects), it just might be worth the effort.