There has been a lot of speculation on this forum regarding how many drops you get per round so thought I'd run a test as the amount per round seems to vary for me from game to game. I will try this out on other maps and try to get some video evidence to support these figures (I tried to record but had problems with video). Nothing over 4 yet, but plenty of rounds had less than 4.
I ran the test from round 20 to round 50 on Die Rise. Training in one room, only leaving the drop if it was a nuke to ensure that I didn't miss any drops. When I am listing the drops per round, I will put LAN (Left a nuke) next to the number of drops if a nuke dropped and I didn't pick it up, but this nuke is included in the number of drops for that round. So, for example, 3 drops LAN is 3 drops, not 4. The nuke is already included in the number of drops.
21 4 drops - LAN 31 4 drops 41 2 drops - LAN
22 3 drops - LAN 32 4 drops - LAN 42 4 drops - LAN
23 3 drops - LAN 33 Jumping Jacks 43 Jumping Jacks
24 1 drop - LAN 34 4 drops - LAN 44 4 drops - LAN
25 Jumping jacks 35 3 drops - LAN 45 3 drops
26 2 drops 36 4 drops - LAN 46 4 drops
27 2 drops - LAN 37 4 drops - LAN 47 Jumping Jacks
28 2 drops 38 Jumping Jacks 48 4 drops - LAN
29 Jumping Jacks 39 4 drops - LAN 49 1 drop
30 4 drops - LAN 40 4 drops 50 4 drops - LAN
4 is the most common number of drops, but not the only number you can get in a round. I'm not sure what round the rule of 4 is supposed to kick in, but I also have video evidence of round 22 Nuketown - no drops, and round 26 nuketown - 1 drop. Ignore the audio in the second video as there is delay (you can hear the fire sale jingle but if you watch the video it is clear that I only get a x2 that round).