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You can have a chain longer than 25 unconfirmed transactions in the mempool, but the wallet still enforces a 25 transaction chain limit.
From running bitcoind -help-debug:
...
-limitdescendantcount=
Deprecated setting to not accept transactions if any ancestor would have
or more in-mempool descendants (default: 25); replaced by
cluster limits (see -limitclustercount) and only used by wallet
for coin selection
...
If you want to test the mempool policy you can change -limitdescendantcount to a value larger than 64 and see if the 65th transaction is accepted (should not).
Without changing the -limitdescendantcount option you will have to create the transaction chain without using the Bitcoin Core wallet.