Maximum number of Grandmas you can have or any other item

It depends on your motivations for playing but the newest target is 200, and beyond that, there are no imperative reasons to purchase any more. If your only goal is to have everything currently available, then you can stop once you have all achievements, all upgrades, 240 cursors and 200 of everything else, got all the seasonal purchases and clicks, 7777 lucky cookie clicks, and have reset 12.5 quintillion cookies away as there is currently nothing beyond that.

You may optionally want to try to optimize CPS gains by balancing out cost/cps for building purchases in order to make more efficient progress later, but that is entirely optional and may not prove to be worth your effort depending upon what is to be added. Because the cost/cps is better on some of the lesser buildings when compared in equivalent quantities of other buildings, your most efficient CPS purchases will be from some of these cheaper buildings. Essentially you would calculate buildingCost/(cps/building)=cpsCost for your most profitable building and then purchase enough of every other building to get the cpsCost for each building to be approximately the same value as was calculated for the most profitable (because they will have the worst cost/value if you don't try to balance the cpsCost). It should be noted that the cost/value relationships change as you purchase upgrades. Having a spreadsheet helps.

You're popping them correctly.

The odds are low, which isn't quite the same as negligible. You are just the "lucky" demonstration of why each curve has a long flat tail all the way out into the 1000s on the x axis of that graph.