IPv4 addresses are 32-bit, traditionally represented as four period-separated "octets" of decimal numbers, each octet having a maximum value of 255 (that is, 192.32.45.4). This provides for a possible 4.3 billion (4.3 x 109) addresses.
Depending on who you ask, IPv4 depletion is somewhere between "rapidly approaching" and "already here" (the RIRs are already out of IPv4 space). Mechanisms exist to extend the usability of IPv4, from virtual hosting, which can stack thousands of websites on a single IP, to vast privately addressed intranets behind a single public IP, but the fact remains that the number of internet-connected devices will at some point outstrip the number of IPv4 addresses available.
This will become exacerbated by the trend known as the Internet of Things (IoT). While it could be argued that the jury is still out...