The more ports you have open on a system the larger the attack surface is. This means the more chances a malicious attacker has to get into your system and do something bad.
It is similar to having holes in a boat - the fewer, the better.
Because it is possible for an unauthorized user to gain access to those unused ports.
There's no limit to the number of serial ports. The maximum number for the traditional ports is 2 (COM1/COM3 and COM2/COM4). Plus you can attach additional serial port adapters to a USB port, up to 127 per controller.
3, 4, 5, 6 it depends on which one you buy. There is really no limit to the number of ports.
There isn't really a limit to the amount of ports, ive seen a port 9999 before, I often use high number ports that arnt assigned to anything. The maximum number is 65,535 as noted in the link below.
Barcelona, Genoa, Naples, and Piraeus are important ports on the mediterranean :)
In LAN we can add as many system depending upon the number of ports in switch and how many switches you are using.
Registered Ports
The minimum number of ports on most brands of switches is four. While there is no industry rule that limits the number of ports on a single switch, practical application has shown that a minimum of four ports is about right.
Ports 135 and ports 137 tell an attacker that the user has a Windows OS system. These ports are used by hackers.
Important pre-colonial ports of India included Barbaricum, Korkai, Muziris, Barygaza, Arikamedu, and Kaveripattinam.
65536 virtual ports. Port number is a 16 bit number, so there are 2^16 (65536) ports for a NAT router.
wiki
Blockades! They blockaded the South's naval ports limiting their trade.