Is there any limit on number of rows that can be created in a Lookup table?

1) For a custom lookup table, please note that the limit is 5000 rows in total. Details can be found at:

https://docs.oracle.com/en/cloud/saas/marketing/eloqua-user/Help/Apps/ContactWashingMachine/Tasks/AddCustomLookup.htm

2) For the data tools' lookup table, there is no document mentioning the hard limit of the data allowed. There is only a hard limit for the number of characters a lookup table field can take which is 250.

However, by no means we are indicating it's limitless to populate your lookup tables and it's wise to separate them into a reasonable amount of data within each table for performance consideration.