I am in NZ so a little different . but I have alot of bikes , the deal I did with my insurance company was seeing as I figured they could only steal 5 maybe 6 at any time . I agreed to that as maximum theft cover.
They cover all the bikes for fire etc under 1 cover with a list of bikes ,(changeable for a small fee) they also only pay out max of $8k on any one bike.
If you explain the situation and shop around you will get the deal you want, the first thee insurers I tried wanted $800 per bike per year, it currently costs me $950 per year for 50K cover on all 13 bikes in total