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)
My understanding is that this minimum you speak of is set by miners using the fee-filter which can be altered without making changes to Core.