Upper and lower bounds of the amount of bitcoins?
Yes, there are 21 million whole bitcoins, each divisible up to 10^-8.
Yes, transactions for fractions of bitcoins are quite common, just as transactions for fractions of dollars are common.  I could send you 0.00000001 BTC if I were so inclined.
I can give you 1 dollar or 100 pennies, but the existence of pennies doesn't really influence the exchange rate of the dollar.