HI THERE ..
i was wondering can you use boost 11 for multiple instrument track at one time?
im  only able to use boost 11 to mix down  one track at the time,or what do you reccomend to use?

Sure you can.
By "I'm only able to use Boost 11 to mix down one track at a time", what do you mean? Can you not insert the compressor in multiple tracks for some unknown reason, or is it that your computer stutters when using more than one instance of Boost 11?
You can also send your tracks to a bus which has Boost 11 inserted in it's effects (FX) bin and the compressor will work on all the tracks sent to that bus.
Or you can select the track that has Boost 11 in it's FX bin and go to the top, Process>Audio>Apply Audio Effects (or something like that) and the effect will be permanently applied to the track and you can choose to have the effect removed from the track after the effect is applied.
You may want to copy the track first just in case it doesn't sound as good after application as you thought it would. You can always Undo the process before you close SONAR, too. 
 
 
 

To add to what Greg wrote,
Put Boost 11 in your Master Bus. Your track outputs should be going to the master bus anyway.
This is where it's most effective. You'll be passing the entire mix through it, and more than likely, you will want to dial some of the individual tracks down a bit.
I also like to put VoxengoSPAN in the master bus, after Boost 11. It gives me an idea of which frequencies may be too hot and helps me keep from overcooking the mix.

boost 11 is also very taxing on your computers RAM so use it wisely my friend.
go as far as you can but if you start noticing drop out or shuddering realize that boost 11 uses look ahead technology and burns up fuel  :)