In Settlers 2, what kind of ratios should I have between buildings? Like how many sawmills per one woodcutter, how many mills per bakery, how many shipyards per harbor, how many wells per farm and so on.
edit. I found out that 2 woodcutters - 1 forester - 1 sawmill should work. How about those others?

It is hard to give exact numbers. The core reason to this is that the way you've constructed your routes and the placement of the buildings will effect the timing of goods. This isn't as critical with a Forester as the only problem it has is a possible lack of space for more trees, but transportation of logs is already a thing where placement matters.
I have some kind of a rule for Woodcutters & Foresters, that being 5 + 2, sometimes 7 + 3. One Forester seems to be able to work over twice faster than a single Woodcutter. Also, I often quickly have three Sawmills running in the game.
Farms + Wells is a different story, because a Farm is not the only place where water is consumed. Most often when you notice your water level is going down instead of increasing in your headquarters then you know you need a Well.
In many cases you may want to go for "over-production", ie. having more in storage than you really need at the current phase of each game, because this allows you to be a little bit quicker. Ie. by over-producing logs, blanks & stone in the early game you allow your construction to be quick. Also limiting to bare minimum for a while will actually result in faster expansion and you only need reprocessing of many things only once you've constructed mines. As you've over-produced until that point you can keep on rapid construction and your builders don't need to wait for more stuff to come.
The only limitation you may have is that in some maps you may start off with limited resources & not having the tools that you need. But that simply changes strategy to a more minimalist line...
Why I got into this kind of general topic is that it has a big effect on how much you build. Reproduction goes pretty much one-on-one (ie. one Iron Smelter for Armory, another one for Metalworks), but it is good to get the raw materials as much as you can as fast as you can (ie. you are likely to run out of Coal if you have enough Iron ore & Gold). The only raw material that you will not struggle with is water.

There's actually at least infinite materials: Food (excluding fish), water and lumber. As long as you build farm and sawmill before you're out of stone, you will have infinite amount of both lumber and grain. Build mill and bakery and you have infinite amount of bread, build pig farm and slaughterhouse and you have infinite amount of ham too. I think that if you "build" a forest you have infinite amount of ham using a hunter.
I like to build a couple of "lumber farms" where I use two sawmills, three woodcutters and two foresters, adding woodcutters and foresters works too, but try to keep foresters "inside" and woodcutters "outside" so that woodcutters can chop down everything.
I seem to struggle with lumber always, mostly because I still don't seem to build sawmills fast enough and I'm out of planks when I build 30-50 places at once (I try to have at least 25 farms in each game).
With mines the ratio seems to be two coal mines per every iron ore mine and one coal mine per every gold mine. If those are next to each other, then it's crucial to build mint as close as possible since gold is totally useless outside of mints. Same with iron ore, totally useless without iron smelter.

IIRC planting forest = only more trees, not animals. Not that it would be such a big deal when you have pig farm.