Farms can have different makes and models. I also think that there is a lot of on board control.
Shipping is also rather expensive, so your are probably going with the nearest factory for everything. (each blade needs a semi with an oversize load permit, and "chase cars" both in front and behind with the right lights and signs)
Of course if a turbine fails in 5 years (I don't know what the warranty is, so I'm going to use 5 years) they might replace it with one from someone else, but that isn't common.
I don't know how much is onboard controls. However someone is feeding the weather instructions in, and when demand is low someone is telling a few to shut down. They also do remote monitoring for issues that maintenance needs to fix. That connection just needs an upgrade, along with some new software for the onboard controls and it can be done offboard. (this may not be easy, but compared to a turbine it is cheap)
At the extreme, I am familiar with farms that have been expanded over 25 years and have a huge variety of builds.
I am also aware of a number of farms that have at least 2 different sized turbines.
I'm reasonably sure that all wind turbines in my area come from Siemans. (they have a factory in my state, any other make would be shipped in from a considerable distance)
The utility might own wind turbines from different manufactures, but they are not in the same wind farm. I suppose there might be places where two wind farms border each other that you want different devices, but for the most part you can optimize each wind farm individually with no need to worry about manufactures.