Can I use a different CHN/CSS number for investnaija

Yes, you can legally have more than one CSCS/CHN account in Nigeria, because each stockbroker can open a separate CSCS account for a client. So what InvestNaija told you is not entirely unusual operationally.
However, your concern is valid: having multiple CHN/CSCS numbers can create management complications if not handled properly.
The key distinction is this:
CHN (Clearing House Number) = your investor identity under a particular CSCS account.
CSCS account = where your shares are warehoused.
You can have:
one investor,
multiple brokers,
multiple CSCS accounts.
The problems usually arise when:
your names differ slightly across accounts,
your bank details differ,
your e-dividend mandate is incomplete on one account,
or you forget where certain shares are held.
That can affect:
dividend tracking,
bonus shares,
rights issues,
portfolio visibility,
and probate/verification later.
Now regarding Bamboo specifically:
Bamboo primarily focuses on U.S. equities and partners with local institutions differently from traditional NGX stockbrokers. Some Nigerian brokers may not accept an externally created CHN if:
they cannot verify it directly,
it was created under another custodial structure,
or their onboarding system only supports internally generated CSCS accounts.
So InvestNaija asking you to create another CSCS through them is operationally common.
My recommendation:
If you intend to actively use InvestNaija for Nigerian stocks, it is reasonable to allow them create their own CSCS/CHN for you, BUT:
Ensure your name format is identical everywhere Example:
Jeremiah Ochoyoda not:
Jeremiah O. Ochoyoda on one platform and full name on another.
Use the same:
phone number,
email,
bank account,
BVN,
and NIN details.
Register e-dividend properly for ALL accounts.
Keep a personal record of:
which shares are in which CSCS account,
the broker attached,
and the CHN numbers.
If later you want consolidation, you can transfer shares between brokers/CSCS accounts through stock transfer processes.
Many experienced Nigerian investors actually maintain:
one broker for long-term holdings,
another for trading,
another for foreign stocks.
So multiple CHNs themselves are not the real danger; poor record management is.
One more important thing: If InvestNaija told you your Bamboo CHN “does not exist,” it may simply mean:
it is not visible within their integration system,
not necessarily that the CHN is fake or invalid.
You can independently confirm your CSCS details directly with Central Securities Clearing System or through the originating broker.