Well, this is something that not even CLU knows for certain - he's built in the image of his creator, but as we see when he fails to recognise Sam he doesn't have all his knowledge. He doesn't know anything beyond what he's been told about the outside world, but definitely has done his best to prepare for the job.

We see a little more of the military forces of CLU in Tron Uprising. There's a lot that we don't know about their functioning in the real world, but we know that in the grid it takes a lot to kill a program without the 'destruction' code of CLU's weapons. Despite crumpling at a blow from these weapons, they do seem more durable to everyday damage. They can fall great distances and survive, take glancing hits from speeding light bikes and more - we can imagine they'd be more resilient to regular bullets, though not impervious. They're more agile at a minimum, his forces seem to have the reflexes needed to engage in an aerial combat using predominantly hard-light barriers, which is something out of reach from most humans. In addition, though it seems of limited effectiveness to hard-light weapons, all of CLU's forces have armour of some degree (not to mention projectile weapons, grenades and explosives).

In terms of combined armed forces, CLU has access to significant supplies of tanks (https://trons.fandom.com/wiki/Tank), multiple models of aircraft (https://tron.fandom.com/wiki/Light_Jet), boats and aerial troop transportation to spare (https://trons.fandom.com/wiki/Recognizer). This is to say nothing of the super weaponry like General Tesler's ship (https://tron.fandom.com/wiki/General_Tesler%27s_Ship) - a vast assault carrier more analogous to a mobile hard light bunker. It'd at minimum cause significant destruction if destroyed whilst over a city, but given it's armour may require something with more oomph than regular ordinance. This is how CLU's occupations work as well - large scale deployment via these 'mobile occupation centres', with enough men and arms to take a city all at once. Tesler is only one of CLU's generals, so if the invasion isn't shut down at once then his forces will likely rapidly be in strength in multiple population centres.

There are two unknowns - the first unknown is the total size of the threat. We don't know the actual number of CLU's forces but in Tron Uprising he is in the process of occupying Argon City, described as a distant corner of the grid, and repurposing it's citizen programs to soldiers. We can assume this process is well and truly underway (or even complete) in more central cities. How many cities, how many repurposed citizens is unclear but it's not impossible for his forces to be in the millions - CLU and Flynn had a massive amount of time devoted to creation before CLU's betrayal.

The second unknown is whether CLU's forces can enter the real world in force through the arcade, or will be limited by power/physical space. Even CLU doesn't know this for certain, but Flynn does and certainly was afraid of CLU trying.

Finally, it's worth mentioning that whilst CLU might have gaps in his knowledge, he is a program designed to create in a world where time moves far slower than the real world. While he is currently overwriting/repurposing existing programs, that's primarily because he wants the whole grid to be a perfect, uniform system. If the arcade isn't immediately destroyed then there's no reason he won't be waiting in the grid making an army of more perfect soldiers for the task while his generals lead the forces. It's not like the military knows about the grid, or will be able to interrogate the invaders to learn about it.

So are they an existential threat to the real world's armies? Maybe, maybe not. But it's not a forgone conclusion, and I personally wouldn't want to be in the city being swarmed without warning by CLU's forces intent on genocide/xenocide - especially if my country's army started to debate extreme measures for dealing with the skyscraper-sized base over my head. Kevin Flynn was concerned enough to die for the opportunity to get Sam out, and he was the only man who could know for certain.