Could you tell me how many days in a year I can let a property elected as my primary residence? 

Arthur Weller replies:  

You can let out your property elected as your primary residence for as long as you like but it may impact on your ability to claim principal private residence (PPR) relief. See HMRC’s Capital Gains manual at CG4311, where HMRC explains three different levels of renting out one's home. At the lowest (first) level, PPR relief is not restricted at all. At the middle level, PPR relief is restricted, pro rata, but lettings relief may be claimed if the landlord lives in the property at the same time as the tenant. At the third level, no PPR relief is available for that part of the house rented out.