I dont like to have too many tabs open, slows down the browser a lot. So usually not more than three...
As a heavy tab user (See above.) I can say it won't slow down as much as you think... Tabs only noticeably slow down the browser if you exceed the amount of RAM on your computer and it starts swapping in and out of the page file/swap file. If you don't do that, you are pretty much in the clear.
This is what Conky tells me when Opera has 65 tabs open:
Opera's RAM usage is at 1.5GB because of the tabs and is still using less than 5% CPU. That 5% is only because of the additional things I have running in it non-stop, and not necessarily because of idle tabs.
tl;dr - Feel free to do as you like, if you want tabs, use em. It won't have as adverse an effect you think it will.
I generally like keeping my number of tabs open to a minimum. Currently I have Twitter, Serebii, and PokéCommunity open; each a pinned tab. On a regular day I'd usually have PC, Tumblr, Twitter, Youtube, and Google open, but if I'm not using them I'll probably close them off.
As of currently, I have 11 tabs open, being Facebook, 4 Pokecommunity pages, 5 Bulbapedia pages for my nuzlocke to know when to evolve my Eevee and finally YouTube, watching some Minecraft Tekkit let's plays on my second monitor.