This might not be how you do it and might not work for you.. But it cant hurt..
If your running like a linksys or something goto 192.168.1.1
Password Admin.
Then on setup
Number of Address: to 1
Change DHCP Address Range: will stay the same.
hope it helps.
Yes it can.
By limiting the number of DHCP addresses you can assign, you are limiting the amount of computers which can have dynamic IP addresses. DHCP assigns IPs to computers which "ask" the router for the IP, if you limit the amount which it can assign you are saying "after one address has been assigned, don't allow any others to have one", seeing as most DHCPs refresh after a certain time it may even change which device is allowed to connect depending on what order the devices try to connect after the refresh.
Static IPs are almost always set up on the computer - You are telling the router "I have this IP", use Gadgethm's link for various different guides on it. Make sure you are not using an already in use IP though.