@frosted1414 is correct, the cap is 2bn, I made that my mission after I hit the igc cap just for gits and shiggles. Like @Willy9292 I mostly used clay courts but use the tables in the link he posted to utilise all the different currencies to the max. The best mix will depend on which characters you can afford to send on what missions in case you need them for other tasks.
Does anyone know if there's a cap on krustyland tickets? as I'm nearly up to 2 million of those already, it'd be great to max those out if possible.