Question about percona xtradb replication (SST / IST )

From Doc :
Each node contains a cache, ring-buffer, (the size is configurable) of last N changes, and the node is able to transfer part of this cache.
Obviously, IST can be done only if the amount of changes needed to transfer is less than N. If it exceeds N, then the joining node has to perform SST.

What is the notion of N changes ? (N queries , N records ??? ). How can I modify this value ?

The N is the amount of transactions that can be stored in GCache. The bigger transactions the less of them can be cached. You may increase this cache size on disk if necessary.