How many gyms above 10 should one hold to have as back ups?

It depends on their average turnover in your area. I have 3-4 back-ups atm.

I try to always have arround 15 gyms

I have a total of 60. You never know...
:D

2 back ups are usually enough for me.

Wish I had that problem! Mine get stuck in gyms. I've lost 4 of mine now for weeks at gyms I drove by and attacked and don't remember where they are. 1 has been at a popular park for well over a month, it's not even level 8 last time I checked.

I don't even understand the question. I don't understand how a gym can be "above" level 10 and I don't understand this, "hold to have as back ups?"

I usually have between 12 and 15. However, I can see many gyms from my house so I try to only take the 10th slot in those gyms if possible. Sometimes I need to put in bigger creatures to defend it, but once it is built up I will just try to hold the 10th slot with a Poliwrath, Slowbro, or Wigglytuff.

around 30 gyms (2K+ pokemon)
+ around 5 (bubbel strat for help other player)