Some exchanges have some coins/tokens that others don't have. But if you trade at a regular basis, I think you should focus on 1 or two exchages with high liquidation. That's enough. For my case, I mostly trade on Binance. I also have other another account on Huobi for less popular tokens and an Okex account for OTC trading.
Very helpful answer complete and detailed,for OP this is enough answer for all your queries,i also do almost what @Lagonda says by using Binance as my main exchange and Okex for OTC
But ofcourse its your call on what to choose since having many exchangers gives you more option on what coins/tokens to invest with