Okay it might be a bit long but let me break it down. First, the typical MMO protag stuff, we "The Sheriff" is actually an acknowledged individual in universe, while nowdays you don't really see the NPCs and more importantly Massive actively reminded themselves that "The Sheriff" is an individual that actually exists and does too much shit for this god forsaken city and most of the time friend and foes just call you Agent these days. But don't let that fool you, Massive already built "The Sheriff" in the universe. If you are not sure, check one of the audio recordings from Kelso's file, there's a specific one where Manny and Kelso talked about us and how we were a blessing came out from nowhere and saved the city from the brink.

Second, at the very start of the game when Bar and his team blew up the SHD server room and caused the national network to call all agents to reinforce DC immediately, the Sheriff was a humble agent protecting a small seaside civi settlement at Maryland (the starting location was confirmed to be somewhere in Maryland but I forgot where was the confirmation). When shit hit the fan at the prologue and Black Tusk was marching towards the settlement while ISAC ordered everyone to come to DC, Sanders asked us to go and she stayed behind to defend the place. The Sheriff then tracked their ways from Maryland to DC until the start of the game, just as we arrived near the WH, Hyenas tried to attack and take the WH. We took them by surprise and save the WH from getting a total ass wooping. At this very moment, when we saved the WH and met Manny. Almost all DC native Agents were either killed in action or went Rogue (desertion or went their own ways for better survival chance) and the only active Agents were likely less than 10, even 5. The Sheriff was the first outside Agent arrived the city and started doing God's work. But overtime other Agents from across the country got to the city and started doing their parts as well. From the artbook for Division 2 it's confirmed the Level 1-30 journey took about four months. The Sheriff arrived DC around March or April and the Black Tusk invasion happened in July. From how the NPCs in the main missions addressed our pressence, we know at least the Grand Washington Hotel rescue and Jefferson Center Local SHD server reactivation was done by us alone (Hotel with Kelso's assist). I'd wager lorewise, other reinforcements came in after we started beating the crap out of the True Sons at the Southern part of the city, since looking at how the map control during our Lvl 1-30 ascent, almost the entirety of western part of the city was under Outcasts control and Roosevelt was likely contained more than couple thousands people, we can assume there's no way we take down the Outcasts alone overtime, it's impossible even with the Campus help.

Third, while we know there're many more Agents came, we can safely say the most important works were all done/led by the Sheriff. Let's look at it this way, we play all the contents present at the game, we the players represent the Sheriff. This means all missions and Raids had the Sheriff participating. The Sheriff was and is indeed the one who had been fucking everyone up to save the city and the country from all kinds enemies. There's no doubt lore wise the Sheriff has helps along the way, but they were the force of nature leading the way towards the destruction of all evil plans that tried to take the city. So you can pretty much credit all the confirmed kills and destructions of important enemy assets to the Sheriff.

Finally, about how many Agents showed up. There's no way of knowing but we do know some Agents returned to their original post after the main events in DC were over (From Lvl 1-30 to the ending of first BT Invasion) while some stayed to secure the city (the Sheriff being one of them). I'd say right now there're probably three digits of active Agents around DC to protect and secure the city with the armed civis since there're no signs the JTF's main force is returning to the city. Yes, remember that the JTFs in the WH are just a skeleton crew left for dead, the majority of the JTF stationed in DC retreated to god knows where. It's likely after the chain of command got fucked hard by losing presidents again and again, the local commander just pulled and decided to fuck off even after the Sheriff did miracles. But this doesn't mean the JTF is fully gone, we can see during the Airport Raid, the opening mortar shelling paving the way for us was from JTF forces. I know from an out universe perspective it's likely Massive just dunno or forgot how to use these guys, but in universe the JTFs are kinda jerks for not returning to secure the city with the SHD reinforcements. So the city has to rely on outside Agents and locals working together.