Before you can decide anything else, you need an honest inventory of what people in your house actually watch on. Not what is plugged in, but what gets used, and when. This page is the walk-round.
Start at the front door and work through the house. Note each screen, who uses it, and roughly when. A television that only comes on at weekends and a phone that only gets used on the train are both on the list, but they will not affect the arithmetic in the same way as two televisions that are on together every evening. The overlap is what you are looking for, not the hardware count.
Most households run this through an app on a smart television or on a stick plugged into the back of the set, and from a planning point of view both behave identically: one screen, one set of sign-in details, sorted in a few minutes. Set-top boxes of the MAG type are the exception worth knowing about, because they are registered to the MAC address of that individual box rather than to you. Replace the box and the registration has to be redone at our end rather than yours, so if you are buying hardware for this, buy the one you intend to keep.
A handset used on the commute is a screen in your household inventory even though it is rarely in the household. The question is still the same one: is anybody watching something else at that moment? If the commute happens while the house is empty, that phone can share with the living room television. If it happens while somebody at home has the news on, it cannot.
Every screen is signed in with whatever details the app in front of you expects, and you do not need to understand the difference between the formats to plan your household. What you need to know is that each screen gets its own set of details, and that two screens running at once need two. Keep those details somewhere the household can find without asking you, because televisions get replaced on days when you are out.
New screens arrive constantly in a family: a birthday tablet, a lodger, a bedroom television handed down when the living room one is upgraded. Adding one later is its own purchase with its own dates, so the cost scales with screens in a straight line rather than tapering off. If you already know a fourth screen is coming at Christmas, it is worth deciding now whether to buy it then or wait and bring it onto the same date as everything else.
Plenty of households list a laptop that gets opened twice a year, or a guest room television that only wakes up when relatives visit. Those are not screens in the sense that matters here. A device used occasionally, and never at the same time as the main viewing, can share with the screen it is standing in for. Count the overlap, not the equipment.
A screen that has been working and then stops is a fixable, specific problem rather than a planning one. It is nearly always the app, the network or the particular channel rather than the line itself. Those symptoms are worked through device by device on trexiptv.forum, including what to do when a replacement television refuses to accept details that worked on the old one.
With an app, yes: sign in on the new set and stop using the old one. A MAG-style box is the exception, since it is registered to the MAC address of that specific box, and swapping it is a support request rather than something you can do yourself.
Only if somebody at home is watching at the same time. A phone used on a commute while the house is empty can share. A phone used on the sofa next to a television that is already on cannot.
As many as you have lines for. Each screen that might run alongside another needs its own, which is why the walk-round matters more than the shopping.
Once you know how many screens and which line-ups, setting it up takes about a minute.
