It is 600 requests (combined from list below) per second per account per region

To Quote official doc:

The AWS STS service has a default request quota of 600 requests per second per account, per region. This quota is shared across the following STS requests that are made using AWS credentials: AssumeRole, DecodeAuthorizationMessage, GetAccessKeyInfo, GetCallerIdentity, GetFederationToken, GetSessionToken

Source: AWS STS Quota