Why Reward Size 50 (21 Million Coins)? I have already read the following answer which is not really meaningful:
21 Million Calculation


  Here's a mathematical explanation: 
  Calculate the number of blocks per 4 year cycle:
  6 blocks per hour
  * 24 hours per day
  * 365 days per year
  * 4 years per cycle
  = 210,240
  ~= 210,000
  Sum all the block reward sizes:
  50 + 25 + 12.5 + 6.25 + 3.125 + ... = 100
  Multiply the two:
  210,000 * 100 = 21 million. 


The total number is thus derived from the reward size and in either case does not provide much insight. I am a bitcoin fan but I am concerned about the deflationary risks in the distant future. 

Since it is open source, if there are deflationary problems in the future can adjustments be accepted by the network to adapt? If so what might the possible solution entail?
The figure of 21 Million is completely arbitrary. It could have been any other number and it would make absolutely no difference, as long as the inflation schedule remains the same.

However, the inflation schedule is universally agreed to be one thing that can never be changed with Bitcoin. Any new code that extends the inflation will no longer be "Bitcoin" and will not be accepted by users of Bitcoin.

To make any change, it is not enough for the majority of nodes to modify, all nodes must eventually change. Usually it is enough for the economic majority to accept the change to trigger this, but with a change like this the worst that could happen is a fork to two different networks.