Depends on the Gym Scene in your area, and how many Gyms you take down yourself.
I have 2, and 2 Chansey's. My two Blisseys are from before the Gym Rework so they're level 30+. I haven't powered them up since the Gym Rework, and I have no plans to because after a few hours, my Level 30+ Blisseys are easy pickings for anyone who has decent Fighting Pokémon, and after 9 hours (a regular occurrence with gyms in my area) a Level 5 Account could take down my Blisseys.
But if you are regularly having people taking down your Gyms within an hour of you taking them, then power up a few Blisseys and TM them to have Zen Headbutt/Dazzling Gleam. How many? How many Gyms are you taking down on a daily basis, and so have the chance to put the first defender on? If that's 1 Gym, than 1 Blissey. If that's 5 Gyms, then 5 Blisseys. But I wouldn't go past 5. (even 5 is more than I'd personally go for - I'd probably go for 3 and simply Revive my defeated Blissey and put them into a new gym)