Well, what breed depends on how much space you have, how much milk you want, what is available in your area, and personal preference. Most of the common breeds are friendly and pretty easy to work with, though Nubians are famous for being noisy and mischievous. We kept Nigerian Dwarfs, as they are good grazers, good mothers, and produce a smaller amount of milk which is more easily consumed by a whole family. Ours, however, never turned out to be milk-goats and were just pets.
Saanens and La Manchas are both known for being friendly and good diary does.
Mix breeds can also be used just fine as milk does, since they still produce enough milk for your average family easily, especially if a dairy-breed mix. However, you also want a goat that doesn't need large amounts of food for the milk they produce, which is why a smaller goat may be better for a backyard goat keeper.
Goats should never be alone, so you should have at least two, but it can be fun to have more, just because of their unique personalities and cool interactions with one another. It is like having a group of dogs playing together, and they love to run with one another. Goats that feel like they are in a herd will feel safer (and likely argue a lot too ). Too many goats, of course, will be overwhelming, so two or three is a good start. Getting goats that were raised together will help prevent issues with introductions or anything like that.
Here is one of our two Nigerian Dwarf sisters, I'm pretty sure this is Lily. Daisy was like a third larger then Lily, who was the runt of the "litter". They were quadruplets.