Pinpoint PutEvents operation, what's the max amount of keys?

For the Amazon Pinpoint PutEvents API, there are several fixed limits that cannot be increased: it allows up to 100 events per request with a maximum individual event size of 1,000 KB and a total request size cap of 4 MB. Each application can have up to 1,500 custom event types, 500 custom attribute keys, and 500 custom metric keys. For attributes, keys are limited to 50 characters and values to 200 characters (events exceeding this are dropped), with a maximum of 100,000 unique values per attribute key (though values beyond this limit are stored but not visible in the Pinpoint analytics console). Each event can have up to 40 attribute and metric keys.

Lastly, the rate quota for the PutEvents API is 15 requests per second.

Sources
Amazon Pinpoint quotas - Amazon Pinpoint

Amazon Pinpoint endpoints and quotas - AWS General Reference

Amazon Pinpoint API reference PutEvents