Heads up is simple:
6 / C(50,2) =
0.49%
In a 9-max game a bit more complicated:
Cards can be dealt to remaining 8 players in C(50,2) * C(48,2) * C(46,2) * C(44,2) * C(42,2) * C(40,2) * C(38,2) * C(36,2) / 8! = 50! / 34! / 2^8 / 8! = 9,980,442,105,172,910,000 ways.
And there are 6 * C(48,2) * C(46,2) * C(44,2) * C(42,2) * C(40,2) * C(38,2) * C(36,2) / 8! = 6 * 48! / 34! / 8! / 2^7 = 48,883,798,066,153,000 combinations where at least one of your opponents has AA when you have KK.
In a hypothetical 26-handed game:
After hero's KK is accounted for the cards can be dealt in C(50,2) * C(48,2) * C(46,2) * C(44,2) * C(42,2) * C(40,2) * C(38,2) * C(36,2) * C(34,2) * C(32,2) * C(30,2) * C(28,2) * C(26,2) * C(24,2) * C(22,2) * C(20,2) * C(18,2) * C(16,2) * C(14,2) * C(12,2) * C(10,2) * C(8,2) * C(6,2) * C(4,2) * C(2,2) / 25! = 50! / 2^25 / 25! = 5.84358414459473 * 10^31 ways.
And there are 6 * C(48,2) * C(46,2) * C(44,2) * C(42,2) * C(40,2) * C(38,2) * C(36,2) * C(34,2) * C(32,2) * C(30,2) * C(28,2) * C(26,2) * C(24,2) * C(22,2) * C(20,2) * C(18,2) * C(16,2) * C(14,2) * C(12,2) * C(10,2) * C(8,2) * C(6,2) * C(4,2) * C(2,2) / 25! = 6 * 48! / 2^24 / 25! = 2.86216366265864 * 10^29 ways where you have KK (or any hand that doesn't include an Ace) and at least one of your opponents has AA.
At least for me, the results are quite counter-intuitive.