Is there really a limited supply of bitcoin? I don't get how there's a limited supply of 21 million bitcoin if we can go into the infinite amount of decimal places. 

So for example, you can't go less then 1 cent if we take USD. in BTC, you can have 0.00000001 or however many zeros after the point, not sure but I would assume it's alot. So isn't there alot more than 21 million bitcoins, if we take in decimals into account?
You can't go into an "infinite" amount of decimal places. The smallest unit of Bitcoin is the satoshi, which is 0.00000001 bitcoin.

There will be a total of 2100000000000000 satoshi.