It depends on how they are broken up throughout your home. Typically, speakers are installed as pairs. So, two speakers per area or room. This would be a standard stereo 'pair'. (did you already know this?)
Assuming you have four areas in your home with speakers to add up to 8 speakers. If so, then you get into a long list of other questions.
Do you want them all to play the same thing?
Do you want all the speakers on at the same time?
Are there any volume controls in place in the walls?
Do you want them all at the same volume?
Are they in just 4 rooms, or scattered across 8 areas?
Assuming a 4-room setup of 2 speakers each, then you have a fairly straightforward setup, and I will assume you don't have volume controls for those rooms, but would like volume control available to those 4 rooms. They will all play the same thing. This is a cut and dirty straightforward setup for audio distribution it will play background music at enjoyable levels, but won't be crazy loud:
AMPLIFIER: Any 60 watt or higher model from eBay (used is a great value on quality gear) -
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If you already have an existing AV receiver, it may work, but be aware that you aren't using 'surround sound'. This is NOT a surround sound setup, it is a distributed audio setup and must be treated very differently!
SPEAKER SELECTOR/VOLUME CONTROL:
https://www.ebay.com/itm/Niles-Audio-SVL-4-Speaker-Volume-Control-System/224308361706
Be aware that these tend to work forever, but if you go shopping online, make sure all the connectors are on the back! There were a few cheaper models on eBay, but they didn't have all the connectors, so I linked to one which will work completely.
SPEAKER WIRE: You only need a few feet of 16 or 14 gauge speaker wire to go between the amplifier and the speaker selector. So, hit up Home Depot or Lowes for this and buy a few feet for 5 bucks or less. Don't go to Best Buy on this one. Don't go online either.
SOURCE: The source is entirely up to you! The amplifier uses standard RCA inputs which means that anything that has, or can be converted to, RCA outputs using line level audio (or close) should work. You can go as simple as wiring your phone directly into the amplifier using a aux cable like this:
Be aware that Bluetooth isn't a technology that covers your entire home, so if you walk out of range, Bluetooth receivers won't work well, but you can certainly pick up a Bluetooth receiver like this and plug it in:
Amazon and plenty of places has articles about Bluetooth receivers.
You can also opt for a unit which operates via Wi-Fi. It sits on your home network and your phone can communicate directly with it to send it audio. Likewise, it can play audio directly off of it using internal apps. So, that would be stuff like this:
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Sonos is a huge go-to brand because it offers a premium experience. You can run apps on it like a smart tv. So, Pandora, XM, internet radio, stream from your phone, etc. Looks like they have over 100 options...
Learn about all of the streaming services available on Sonos. Browse by features, types of content, and more. We’ll help you find your next favorite service.
support.sonos.com
Not an inexpensive solution...
Introducing Port: the versatile streaming stereo upgrade from Sonos. Enjoy streaming music, podcasts, audiobooks, and internet radio on your existing stereo over WiFi with just one simple connection. Control your audio with Sonos app or Apple AirPlay 2, and easily expand your system to more rooms.
www.sonos.com
There are also stand alone players, which can just play music. You can set them up with music services, but they may not be perfect at streaming from your phone.
Alexa devices for example (actually I haven't tried streaming from my phone!)...
