Here is an old but relevant article. The author of the article was one of the National Guardsmen who was hit with a pre-emptive strike back in 2005, so the website is out of date but most of the information is not.
http://savvysurvivor.com/extremism.htm He puts the estimate at around 45,000 fighters if and when key events happen that make it clear the game is on. Pre-emptive takeouts do not count.
The government has been able to do mass arrests of up to 25,000 people that I know of with no immediate organized resistance and very little retaliation. That was in Los Angeles when known and suspected gang affiliated people were arrested in large numbers so as to be "off the streets" while the Olympics was in town. The operation was ongoing and some estimate the numbers arrested was closer to 50,000, although the figures get muddy since apparently several arrests were multiple arrests of the same people.
From Wikipedia:
Operation Hammer
As a vast majority of those arrested were never charged, Operation Hammer was roundly criticized as a harassment operation whose chief goal was to intimidate young black and Hispanic men. In a PBS interview, when asked whether the local people in the minority areas expressed thanks to the police for their actions, he responded:
Sure. The good people did all the time. But the community activists? No. Absolutely not. We were out there oppressing whatever the community had to be, whether it was blacks, or Hispanics. We were oppressing them. Nonsense. We're out there trying to save their communities, trying to upgrade the quality of life of people...
By the time of the 1992 LA riots, minority sentiment against the police was pretty bad and boiled over into resistance operations with both a political aspect (Maxine Waters) social/entertainment industry aspect - Ice-T and many others encouraging acts of resistance - and then of course, on the streets, many operations carried out from the Reginald Denny beating to some highly sophisticated raids on firearm wholesale business warehouses, and a raid on a laundry that specialized in police uniforms. As we know, that "resistance" led to attacks and operations as far away as Canada, but eventually settled down as the various factions negotiated their new pieces of the political pie. Many would argue that "uprising" is what led to Democrats taking power in the next election.
On our end of things, I don't think we are seeing coordinated raids on more than 20 people at a time without alerts going out and any further raids either not being planned or being called off due to the high threat level of retaliation or resistance.
Personally, I see the problem as being that the true patriotic militia movement leadership right now is less than 500 people, and for the most part not particularly well coordinated and spread out enough geographically that it may be possible for all or most to be taken down in a relatively short period of time if the opposition prepares and coordinates a large enough operation and public sentiment within the next level of the movement -those 45,000 capable and motivated fighters representing the "biggest problem 1%" of the NRA. That's the crowd that we see being recruited into the false flag and sellout organizations and if that crowd gets turned against resistance leadership, then you have situations like Hollingsworth and his people turning on Charles Dyer and his small crew or Lackomar and Savino's Michigan militias turning on the Hutaree.
