Question about the tabletop game. How many of the same units can you field? Is it a properly set rule or does it depend on how casual or competitive the game environment is? I'd like to put together an imperial guard cavalry army but I don't know if I have to bring in multiple different units like rough riders, death riders, mukaali riders, etc.
You're limited by the Force Organization charts and by points limits. Just going off of Wahapedia, if you're doing a Battalion, you can have 2-3 HQs, 3-6 Troops, 0-6 Elites, 0-3 Fast Attack, 0-3 Heavy Support, 0-2 Flyers, and a Dedicated Transport for each infantry choice you have. So just off the basic list, you can field up to 3 Riders in your fast attack slot, 3 Death Rider Squadron Commanders in your HQ slot, and 6 Death Riders Command Squad in your Elite slot plus an additional one for each HQ that don't count towards your slot limit. This gives you a max total of 15 cavalry units (69 total cav models) for 1500 points or 1515 points if you upgrade the DR Commanders to have plasma pistols.
There's also other FO charts for that expand on a particular slot at the cost of reduced allowances in the others as well as not getting extra command points for you to utilize. You got the Vanguard for Elites, Outrider for Fast Attack and Spearhead for Heavy Support. Depending on your points limit, you can also take different/mix-and-match detachments as an option. If you want to field a fuckton of cavalry, the previous paragraph grants you the most cav units, but there's a lot more options now compared to my entry in 5E.
EDIT: I should mention there is a Brigade detachment that lets you bring 3-5 HQ, 6-12 Troops, 3-8 Elites, 3-5 Fast Attack, 3-5 Heavy Support, 0-2 Flyers and a dedicated transport for each infantry. So you can bring even more cavalry, but you're getting into Apocalypse scale games then with how many points you'll need to field to max out your cav.