What can I expect from a hair transplant at this stage? I'm 27 (Photos)

Based on your photos, you appear to be around a Norwood 5 or 6, the donor area doesn't look very abundant though this is just an assessment based on pictures, and a detailed examination with a trichoscope is necessary. However, I would recommend using your beard hair as a donor source as well.
Even so, this isn't a case of baldness that can be resolved in a single operation, two sessions will be required.

Thank you very much for sharing the pictures with us. As you can see you are now officially a Norwood six with no hair in your frontal third through the vertex and in the crown. You are at the point where you need to decide are you going to be bald for the rest of your life or are you going to get a hair transplant. In your case we could start with the first procedure addressing the frontal third and getting about 2/3 to 3/4 of the way back get into your crown. And then a year to a year and a half later we would be able to complete the rest of the hair transplant filling in your crown. It is important to understand that when you are a Norwood six you are going from zero hair on top of your head to approximately 30 to 35% density.

One session will not be enough for you, since the area of baldness is quite large, in one session it will be transplanted approximately 4600-4800 grafts using the FUE method, thereby closing 70% of baldness. We will restore your anterior line, temporal receding hairline and also close up to the beginning of the crown. The crown will remain for the second session, since at least 2500 grafts are needed there, and you can't take that much from the donor zone at a time if we don't want to damage it. Therefore, in your case, there will be an excellent thick result of an 70% filled head.