Of course, all the answers that have been posted thusfar are FAR too low. I'd say it's on the order of billions, given the sheer number of people in the Milky Way Galaxy (not to mention Pegasus, where non-slave humans regularly use gate travel).
I think that if we're restricting it to Earth, Pearson is on the right track.
However, I think that's a very, very conservative estimate. Firstly, 4 seems to be a fairly low number for an SG-team. SG-1 is a light recon team, not geared up for assault (though they do also do special forces work when called for). Most other teams seen in the show tend to be a minimum of 6 people, though SG-3 (marines, in a security or fire support role typically) tends towards 8-10 men. I'd peg the average SG team size at 7.
I think the number of personnel at Cheyenne is low by an order of magnitude: 100 people for an entire military base, comprised of everything from janitors to network support staff to guards? 100 people MIGHT do for a single shift, but I'd say the total number of personnel at any given time would trend more towards 500-750.
Most of the rest of Pearson's numbers also seem to be extremely lowballed. We saw an entire wing of F-302s at the Antarctica battle. They had to be constructed, maintained, prepped, controlled by ground sources, and flown. Right there there's at least a hundred non-SGC Air Force personnel, not counting backups or other shifts.
Then there's the humans who've used the gate (from Earth) who weren't protagonists. At least 30 people from NID black ops knew the whole story, God alone knows how many were involved with the Russian gate program, etc.
Essentially, you have to account for at least 7 full military bases full of people (SGC, Antarctica base, Area 51, two 'permanant' off-world bases, and a minimum of two construction/basing facilities for F-302/303s) and most likely more. Each of those bases will need between dozens to hundreds of specialists plus security personnel.
Given all of this, I'd assume that there are at least 3,500 Air Force or retired Air Force personnel who know a significant part of the whole story. Then, adding in civilian oversight (and the leaks that are always involved in modern government) and manufacturing personnel (US) I'd peg the 'significant knowledge' number at 5,000.
Other countries involved in the oversight wouldn't have as many personnel, but the sheer scale would have to take the number up to close to 5,750.
Next we must consider Atlantis. I'd peg the number of people in the series launch at about 75. At least one of them got permission for a significant other to be informed. This indicates there may have been others. Let's round to 80 to account for that. Assuming a low failure rate when prepping for the mission, we can take this to 100. We can cut the Atlantis number off there (as of what I saw, to mid-season 2 or early season 3).
I have no knowledge of SGU, so I must assume Pearson's guess of 100 is accurate.
This gets us up to ~6,000.
So there are about 6,000 active and retired people who know about the gate in general terms, or at least that humanity is actively exploring the galaxy and has encountered hostile races.
Next, we must consider the SGC's training program. As we see in one SG1 episode, there is a large pool of SG personnel who aren't active on a team (trainees who have passed, but not yet been assigned). This pool is likely tapped when an SG team suffers casualties or other personnel losses, when a team member takes leave, or when new teams are created. I don't think a pool of 25 is unreasonable to assume.
For each member of that pool, there are doubtless several who were NOT up to snuff - the SGC is an elite command, accepting only the best of the best. For many elite groups, especially those which feature very stressful purposes or require unique skillsets, a 25% pass rate is considered good. So we can assume that for every person who enters the pool, 3 people don't (but get far enough in training before washing out to have a good idea what they were being tested for).
This pool has been drained and refilled constantly as the series has progressed.
Therefore, I would put the absolute minimum number of people with some knowledge of the Stargate Program at approximately 7,000. My confidence in this number is roughly 80%. To approach 100% confidence, we must drop the minimum to 6,250.
So a minimum of 6,250 to 7,000.
Given that there are now 5 ships, including 2 Chinese (? I believe, from Universe?) this estimate needs to be revised upwards.
Given the personnel requirements we've seen on the ships on SG-1 (which I estimate to be approximately 250, given 16 F-302s each, to account for observed crew, required support crew, security forces, etc) and the required Chinese facilities, I estimate the total must reach at least 8,500 - 11,000. The actual total may be much higher, as we don't know how many other countries are following China in developing their own 304s, or how many people China actually involves in their 304 construction process.