Heya,
Well, starting out, start with one light. If you go with Yongnuo, I'd go with the 685 at first. It's very good. It works on the 5d classic just fine too, and does HSS, ETTL, etc, appropriately. I use mine on a 5d classic, it's great. But starting with one light is a great way to learn everything, as bouncing will be your most common use of the speedlite inside, with ETTL and some FEC compensation. Outdoor, you're more likely to direct flash with ETTL/HSS and FEC dialed in as fill light. One light does this fine and you can learn from it.
Multiple lights is for when you're setting up a stage/studio/event/etc. The beauty of the Yongnuo is that you can add 560 series or more 685 series and they all function off the same triggers and all of them have built in wireless receivers so you don't spend a lot of money on triggers/transceivers/batteries, and just get flashes for your money.
Alternatively the Godox system is very good, probably the best right now. You could get the equivalent Godox flashes in the form of Adorama Flashpoint (Flashpoint Zoom R2 TTL has the HSS/TTL, built in receiver, etc and the R2 Manual is the same thing but without ETTL/HSS, and is a manual flash but has the built in receivers, etc; these are Godox flashes rebranded under Flashpoint so you get Adorama's support) or Cheetah (Cheetah V850, etc) if you want some support in the US. Might cost a bit more, but you get them in the States with warranties and support in the States which is very good.
I started out with the Yongnuo manual flashes. I traded them all in for the 560 III series Yongnuo's because of the built in receivers, that is a big deal to me, and the ability to use the TX-560 which lets you use 6 groups of manual flashes remotely and remotely change their power levels, zoom, etc, turn on/off groups, etc. Love the system. So simple and convenient. The III & IV 560 series are virtually the same, so doesn't matter which you go with. I have a fleet of 560 III's. I use them for setting up multiple lights and control them remotely with a single controller (TX-560) from the camera (works on 5D classic just fine!). I used to have the 565EX flashes as well, but I sold them in favor of the 685. The 685 works properly on the 5D classic with ETTL/HSS and the 685 can work on the 560 protocol or the 622 protocol, so I can add it to my manual fleet just like another flash, etc, but I mainly got it for the on-camera ETTL/HSS usage for spontaneous shooting where there's no setup time, no metering, no modifiers, etc. If I were doing it all over again, I'd probably spend a bit more and get the Flashpoint R2 system simply due to the warranty/support. But that said, I'm still very happy with my Yongnuo system, it's bullet proof for me for years now.
Very best,