There is no one answer, as everyone's incoming signal level is different. Some homes the signal at the ground block is 0db, some it's +5, +7, +8, +10db. Some are lower like -4db. It can be anywhere, as the power level at the tap varies by system, and the loss thru the drop cable. Some homes have a 30 foot drop cable, some have 250 foot drop cable, that makes a big difference on the signal at the ground block along with the level at the tap.
So you need to know what is coming in to determine how much loss you can get by with. If you have Comcast HSI, you can look at the power level at the modem to get a rough guess on what's coming in. http://192.168.100.1
The slgnal levels will also vary from the low frequencies ot the upper frequencies. That's why users that had no signal issues with the analog channels are now seeing issues with the higher frequencies that some of the digital channels are carried at. The analogs were at the lower end of the frequencies, up to 550mhz or so. Digitlal on most systems are up to 750 or 860mhz, thus more loss and lower signal levles at those higher frequencies.
Some homes you can get by with a 8 way splitter that has 11db of loss. If you add a 2 way splitter for a cable modem in front of that, and then your splitter loss is 14.5db. Those levels will cause problems if your incoming signal is around 0db, but if your incoming is hotter like +5 or +8 than you might still have enough to drive the set top boxes.
With digital cable, guessing on signal levels can no longer work, the tech needs to know what the levels are across the complete band of frequencies.
Normally a cable modem should be fed off a 2 way splitter (-3.5db loss) first thing coming into the house. That will get you a rough idea where your levels are at. If your level at the ground block is 0db, and you place a 2 way splitter to feed the cable modem, then at each port the level is down to -3.5db. Feed one side to a 4 way TV splitter (-7db loss) and add some loss for the coax. (-3 db for 50 feet) and your signal at the outlet would be -13.5. That's getting at the edge of reliable signals for a set top box. In this case you would add a drop amp in front of the 4 way splitter, and boost your signal back up +15db. Now your signal at the outlet would be +1.5, very reliable for a set top box.
Daisy-chaining your coax and splitters causes issues, that the outlets up front get alot of signal and as the chain continues, splitter loss after splitter loss plus the coax loss might not be enough to drive the last outlets in the chain. Adding a drop amp in this case might be overdriving the outlets in front causing issues with too much signal.
So the answer is, without knowing the signal level coming into the house, and how the wiring is run, there is no set answer. Many homes will have enough signal to drive a 8 way splitter (-11db), or a 2way (-3.5db) and a 4 way (-7db) behind it. Others might not have enough signal to drive a 4 way splitter (-7db). Techs can sometimes get creative with splitter arrangements to get the proper signal to each outlet. They have the proper test equipment to know exactly what the levels are at each outlet.
A drop amp will help make up for loss in the splitters and cabling. But you can't guess at it. You might get lucky with a 4 way splitter with no drop amp, but a 8 way splitter would almost always need a drop amp these days.
Any more questions with this, please don't hesitate to ask.
A drop amp (booster, cable amp) is a small AC powered RF amplifier made specifically for cable TV lines, and some will work on antenna's also. It is specifically made to cleanly amplify the incoming signal without distortion and adding noise to the system.
Most give +15 db of gain on the downstream, and most have about a zero gain / loss on the upstream signals (upstream is for the set top box talking back to the system for On Demand). Under most conditions, a set top box will have enough power to talk back to the system that you don't need to amplify the upstream, but there are exceptions. Amplifying a upstream can cause noise issues on the upsteam frequencies and can cause issues for neighbors blocks away with High speed internet and On Demand.
The +15 db of gain will make up for the splitter and cabling loss in the home, it can't improve a already poor signal condition. That's why drop amps are not recommended to "band aid" a problem, when something else is causing the initial signal problem.
Here's an example HERE of one that Comcast installers use. These high quality drop amps are not available locally in stores. The ones you see in many stores are true junk, and are only do one way downstream. You will see these hanging on peg hooks in the stores in little silver or black housings.
Any more questions, please ask.