Roughly a million people died when the first Death Star was destroyed, did this cause a disturbance in the force like what happened at Alderaan? If not what figure would be needed to generate the reaction?
I don't remember and can't find any reference to a disturbance in the force when the Death star is destroyed, I will try to speculate a bit. Quoting Obi-Wan Kenobi:
I felt a great disturbance in the Force, as if millions of voices suddenly cried out in terror and were suddenly silenced.
We can see that he refers to the death of millions of people, so probably the death of one million could trigger a disturbance. Although, we know that disturbances in the force don't always happen due to mass deaths, as is shown in this article. Indeed the force underlies all things in the universe, so the famous quote "I have a bad feeling about this" is in fact caused by a disturbance in the force. On the other hand, it is clear that a disturbance can be sensed by someone as particularly strong when the person who dies has some relation to who feels it. For example, when Yoda senses the death of his fellow Jedi carried out in accordance to Order 66, the feeling is so strong to get such a powerful master to his knees. So my point is: a disturbance in the force probably happens at the moment of anybody's death, beeing the force in everything, but is sensed, and sensed with different intensities, by different people. I will say, then, that the destruction of Death star caused indeed a disturbance, which wasn't acknowledged by no one of the Jedis due to their relatively low connection with the casualties.
In addition to the loss of life. The death star laser was powered by huge Kyber cristals which 'resonate' with the force. The destruction of these would likely cause a 'disturbance' in the force.
Never occurred to me before that the death star was basically just a giant lightsaber
My answer is definitely yes, as the Force is created by living things and exists between different elements of the Star Wars galaxy, connecting them. Over two million sentient life forms were killed in a great explosion. The extinguishing of life on such a scale would certainly cause a disturbance, and we know that the Force binds the galaxy together.
Obi-Wan explained that, "The Force is what gives a Jedi his power. It's an energy field created by all living things. It surrounds us and penetrates us. It binds the galaxy together."
Yoda said, "Life creates it, makes it grow. Its energy surrounds us and binds us." All life creates it, regardless of whose side you're on or anything else.
He added, "You must feel the Force around you; here, between you, me, the tree, the rock, everywhere, yes. Even between the land and the ship." This illustrates that the Force is everywhere and connected to all things, so a disturbance would travel.
Who actually felt this disturbance in the Force cannot be answered because it wasn't referenced, but I cannot think why one would not be caused.
"All energy from the Living Force, from all things that have ever lived, feeds into the Cosmic Force, binding everything..."
―The spirit of Qui-Gon Jinn, communing with Yoda (Clone Wars: "Voices")
The loss of life from the Death Star certainly caused a disturbance in the force, but the question becomes who would have felt it.
We see Obi Wan crippled by the destruction of Alderaan, but Luke barely reacts to it because he's so much less sensitive to the force than Obi Wan.
The only two force users we see actually see reacting to the Death Star destruction are Luke Skywalker and Darth Vader.
Luke doesn't feel much, which is unsurprising, since he didn't feel the destruction of Alderaan. (Wouldn't Alderaan's population actually be in the billions, so even accounting for Luke's growth in the force between Alderaan and the Death Star destruction the Death Star disturbance would have less magnitude.)
Vader's reaction to the destruction of the Death Star is understandably one of anguish as he spins off into space in his TIE fighter, though perhaps Sith draw pleasure and power from destructive disturbances when it's not so personal.
Yoda and the Emperor aren't shown in the moments after the explosion, and don't directly comment on their experience of it, but it seems logical to assume they both feel it before news can reach them, since they're attuned to much smaller or more dispersed events throughout the universe (Palpatine's knowledge of Luke, Yoda's experience of Order 66.)
Maybe as this part of the new Star Wars canon is explored, we'll see other characters more attuned to the force reacting to the Death Star from afar - people like Ezra Bridger, Kanan Jarrus, or Ashoka Tano who may still be alive during the Death Star destruction - and have more concrete evidence.