The length of WOTS+ private key is an array of length "len" of n-Byte strings. Meaning, for w=16 and n=32, the len is 72. So the WOTS+ max length of the private key is 72, meaning only 72 messages can be signed.
Now my problem is, when you read about XMSS Keys, it says that the private key is defined by 2^h OTS private keys, h is usually 20 so 2^20 keys. What do I generate first? With the first definition I can only have 72 private WOTS+ keys and the second one extends it to over a milion.
Do I have to ignore the fact of array[len] n-Byte private keys and generate just 2^h WOTS+ keys using the chaining function?