How can i get market cap of specific coin pair in API ?
You can compute it on your own.
Go GET /api/v3/ticker/24hr of the symbol
You have there the volume.
Choose the price of your choice, weightedAvgPrice might be a good fit.
Multiply this by that and you have your market cap.
Hope it helps 
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By market cap i suppose you mean current price x circulating supply? If so, this not available via Binance API
Market cap = circulating supply x current price
you can get the circulation supply (cs) of a pair from binance:
exchange-api/v1/public/asset-service/product/get-products
this is an example with binance client:
from binance.client import Client
client = Client(key, secret)
data = client.get_products()
You can then filter the result and calculate the Market cap.