As Adam answered, the Coinmarketcap.com API has a ton of data that's aggregated from a ton of sources. Also, it's been around for a while and the API is accessible 24/7, so it's definitely one of the more reliable services. I heard they even filter out sources with outlier data, such as price or volume spikes from artificial pumping or commission-free trades, which makes the data that much more reliable (IMO).

The following URL will return JSON with high-level stats for every coin tracked by the API:

https://api.coinmarketcap.com/v1/ticker/

A sample response is as follows:

[{
        "id": "bitcoin", 
        "name": "Bitcoin", 
        "symbol": "BTC", 
        "rank": "1", 
        "price_usd": "10077.0", 
        "price_btc": "1.0", 
        "24h_volume_usd": "9717610000.0", 
        "market_cap_usd": "169661279499", 
        "available_supply": "16836487.0", 
        "total_supply": "16836487.0", 
        "max_supply": "21000000.0", 
        "percent_change_1h": "1.34", 
        "percent_change_24h": "-9.2", 
        "percent_change_7d": "-7.58", 
        "last_updated": "1517379267"
    }, 
    {
        "id": "ethereum", 
        "name": "Ethereum", 
        "symbol": "ETH", 
        "rank": "2", 
        "price_usd": "1077.71", 
        "price_btc": "0.107872", 
        "24h_volume_usd": "4432010000.0", 
        "market_cap_usd": "104880306525", 
        "available_supply": "97317745.0", 
        "total_supply": "97317745.0", 
        "max_supply": null, 
        "percent_change_1h": "1.06", 
        "percent_change_24h": "-7.55", 
        "percent_change_7d": "8.78", 
        "last_updated": "1517379253"
 }]

Some of the properties you may be interested in are as follows:

• "market_cap_usd": Market cap in US dollars.
• "available_supply": Circulating supply.
• "total_supply": Total supply issued/mined.
• "max_supply": Total supply the platform/protocol currently allows for.

I'd check out the Nomics API. It's free, has no rate limits, and gives you more historical data (for free) that CoinMarketCap makes available on it's most expensive plan.

Nomics' free (and paid) crypto exchange data API comes with dozens of endpoints, including endpoints that return sparkline, exchange rate, all-time high (ATH), and supply data. Also included are historic candle/OHLC data for cryptoasset trading pairs on exchanges like Bithumb, Binance, Bittrex, Bitfinex, Bitflyer, Gate.io, Coinbase Pro/GDAX, Gemini, HitBTC, Kraken, and Poloniex.

One notable difference between Nomics’ API and competition is that the Nomics product has no rate limits. For example, CryptoCompare and CoinMarketCap’s APIs all limit the number of application programming interface calls that can be made per minute, whereas Nomics does not currently impose such limits.

Disclosure: I'm one of the founders of Nomics.