It really depends on how much butter you want and how strong. If a recipe calls for 2 sticks, then it doesn't need to be as strong, if it calls for one stick, well, the math is easy enough. Generally, if bud and psycho hash worthy sugar leaf and fine trim, about an ounce per pound for recipes calling for 2 sticks... about 2 ounces per pound for recipies calling for 1 stick (or to really mess you up on the recipes that call for more, lol).
Things to remember:
1 Decarb plant material first
2 Use soy lecithin if you can get it
3 don't cook on high heat, use warm or low in crockpot or a double boiler
4 keep the lid on while the butter/water/bud cooks
5 put in a good amount of water... it cleans the end product. You can actually do the water to butter process a few times to take all the plant taste away, or most of it.
6 make sure there is room in the fridge for whatever you're cooking in so you can cool it and solidify the butter
7 eat small amounts the first time you use it and remember edibles work best on an empty stomach and take most people 30 min - 1 and 1/2 hours to feel it (some as long as 2 + hours), but the effect lasts a lot longer
If you want some help when you're ready, let me know. I've been doing a lot of experimenting with canna cooking using everything from deserts to gourmet meals (early teen jobs were in gourmet resturaunts). It's not too hard once you get a feel for it. Personally, I love using kief/hash/honey oil (non butane) when cooking... makes for some killer food, lol.
Final note, crockpot is your friend for butters and oils or any other fatty extraction that requires heat. It's so much easier to deal with and you can usually find some kind of small, generic crockpot/slow cooker type thing for $10 - $20 USD or so. Just make sure it's big enough to hold the plant material, butter/oil/coconut oil/etc and an equal or greater amount of water. It's best to find ones that have a "warm" setting as well as high and low.