Standard issue US military magazines are 30 round. Different AR's are sold with different magazines, often MAGPUL plastic magazines of 20 round capacity.
As you pointed out, magazines are available in many different capacities. But the 30 round is most common, by far.
At this time, there is no US federal limit on magazine size anyone can possess or use.
There are also 'oddball' magazines, often rotary, that can and do have much more capacity. But I do not know of any '50' round magazines and I do not believe there are any '60's either. That said, I am not aware of ANY nefarious uses of any AR with the 'oddball' magazines, all that I have see from photographs have been the ubiquitous 30 round, metal, <probably> standard issue magazines.
Our associate (in this conversation) is simply throwing numbers out in his ignorance, believing accuracy not important so '50' and '60' sounds better as a 'sound- bite' than 20 or 30. My point was that there is absolutely no reason to engage him / her on this topic and start doing math with other firearms magazines nor compare what other firearm magazines may hold, how long it takes to change them and so forth. Put simply, this gentleperson is throwing out snippets he / she has picked up from [often also badly informed] news outlets, and he / she is no longer discussing anything, merely taking potshots (pun intended).
But, as always, it IS a free country and all are of course free to go down any verbal path, the gentleperson specified also of course. Carry on.
I do believe the original topic was the tragic high school shooting in Florida on 14 Feb. 2018
Brian
The AR-15 is issued with a 5, 10, 15, 20, or 30 round magazine. To my knowledge, 30 is the largest legally sold or can be posessed magazine ANYWHERE in the USA. If you live in the ridiculous states of CA, NY, MD, MA, and CT a few others, it is 10. NJ and CO, it is 15.
So, yeah, 'sup with this "50-60 round magazine" stuff, Sanmo??