"rather many" is correct and usable in written English.
You can use it to indicate a greater number than expected. For example: "I expected there to be a few people here, but there were rather many!".
So he adopted a pseudonym, or rather, many pseudonyms.
His views haven't changed; rather, many Democrats have.
Rather, many farmers expect the government to reward them for giving up the crop.
Or, rather: many bands simply cannot survive because everyone is stealing their music for free.
Rather, many of the matters at stake were routine, and nearly all the votes overwhelmingly carried.
Rather, many insisted that it takes a campus to encompass the college experience.
Rather, many said they enjoyed the feeling of participating in a giant real-time civics lesson.
Rather, many will want to fight — literally — for what they believe is right.
Because Halston dislikes plates and knives and forks, he did not serve dinner but provided, rather, many elaborate hors d'oeuvres.
Rather, many people feel that the lawmakers went too far in their prohibition, challenging Roe v Wade.
"The new melting pots are not large international gateways," Professor Frey said, adding, "Rather, many are fast-growing suburbs themselves".
