You can use 1 API key per trading bot. This applies to most crypto exchanges.
Kraken: additional API key
On Kraken, you can use 2 API keys per trading bot. This allows the bot to make more API calls, which improves performance. If Cryptohopper provides the option to add a second API key, it is recommended to use it.
Sub-accounts
If your exchange supports sub-accounts, you can use multiple API keys per exchange account. Each sub-account has its own API key and acts as a separate trading bot. This is the recommended way to run multiple bots on the same exchange.