After peaking at approx $800 bil in Dec 2017, cryptocurrency market cap gradually decreased (~ 289 bil on 23 Jul 2018). But in contrast, the number of exchanges
increased dramatically during that time.
Market cap is computed by multiplying total supply of all coins times the current prices of each coin. Since most of the market dumped or have gone down in value when compared to their peak prices in December 2017, it is a given that market cap will also decrease. The number of exchanges does not affect the computation of market cap.