Given that the minimum transaction size is 547 satoshis or 0.00000547 BTC, there would be a theoretical maximum of 3,839,122,486,289 units of Bitcoin available for spending
the minimum amount of bitcoin to send has changed a couple of times but last time i checked it was 5 thousand something (5400?) satoshi
also to my knowledge this minimum changes based on price of bitcoin and it is there to prevent spamming of 1 satoshi transactions. in other words in a world where everyone is spending bitcoin, price is much higher and if we are about to spend all the units available this minimum will be 1 satoshi (the samallest amount)
this makes the maximum theoretical amount equal to 2,100,000,000,000,000 (ignoring all the lost coins)