How much shared memory does Postgresql need per max_locks_per_transaction?

I have a Postgresql 10 database with about 300k tables in 23k schemas. I am
trying to upgrade to Postgresql 13 using pg_upgradecluster. This is failing
while attempting dump all the schemas:

pg_dump: error: query failed: ERROR: out of shared memory
HINT: You might need to increase max_locks_per_transaction.
pg_dump: error: query was: LOCK TABLE "a45119740"."activity_hrc" IN ACCESS
SHARE MODE

Is setting max_locks_per_transaction to 300k something that can be done? I
haven't been able to find anything explaining how much shared memory this
might need. The machine has 64G of RAM.

(I understand that I need to change my db design .. I have been backing up
one schema at a time until now so wasn't aware of this problem)

Thanks
David