Number of API calls you can fire on the instance (just in case is it Adobe hosted) will be specified in your contract.
Also it greatly depends on how many web servers have you bought so that the load can be shared.
The number of calls per second depends greatly on what kind of call you are firing, for how long that calls keeps the connection open between the calling server and Adobe Campaign web server, to process and return the data.
Best way to find it out is by performing a load test.