And the last question here: is there a big difference between squeezing a capsule once and twice concerning caffeine content?
How many are too many for a day depends on which you're using. As you suspect, different pods might have different levels of caffeine. Darker roasts tend to have less caffeine, while lighter roasts and flavored coffees likely have more caffeine. From what I can tell, it looks like Nespresso pods ought to vary between 50 and 150 milligrams per pod with the majority of varieties in the 70-80mg range.
Regarding caffeine consumption, please understand that we are not doctors and cannot provide medical advice. Everyone is affected differently by caffeine. If you're looking for your own limit, start at 2 Nespresso brews per day (140-200mg) and work from there. Also, coffee itself isn't generally harmful to people, though sweeteners, including milk (lower fat milks tend to be higher in sugar), can be.
Lastly, regarding caffeine extraction, the answer depends on some variables. Coffee grind size, water temperature, brew time, brew method, and amount of caffeine in the coffee all affect this. For the Nespresso, which uses pressure to draw a relatively short extraction somewhat like an espresso machine, the first brew probably extracts about 60% of the caffeine, while a second brew of a once-used pod might get you 80%. This is just a guesstimate based on what I know about espresso brewing.