I purchased and sold mutual funds in 2016 son have a t5008-doi just enter box 21 ( the proceeds of disposition) or do i need to figure out box 20 ( cost or book value). does it matter??

You do wish to report the cost in box 20...as that money would have been the money you yourself invested.  You do not want to pay tax on your own money...the actual gain or loss is calculated by subtracting the cost from the proceeds, and then subtracting any outlays (transaction fees).  If you do not report an amount in box 20, then CRA will all too happily tax you on the portion which is your own money.

so i purchase a percentage every other week off of my pay - therefore do i add up all those purchase amounts from jan 1/16 to dec 31/16 and enter that in box 20? there are no other costs associated with those purchases.

If you sold all the stock, yes.  If you sold part of your holdings, then still add all the purchases, divide by the number of stocks purchased, and multiply that by the number of stocks sold.