How many NAICS codes can I list in SAM.gov, and does adding more of them ever hurt a bid?

The primary NAICS code in SAM.gov does not decide which solicitations you can bid, and size is measured against the single NAICS the contracting officer put on that solicitation, not against your primary.

FAR 19.102(b)(1) and 13 CFR 121.402(b) say the contracting officer assigns one NAICS code and one size standard that best describes the principal purpose of the buy. 13 CFR 121.402(a) then says a concern must not exceed the size standard for the NAICS code specified in the solicitation. FAR 19.102(a)(3) fixes size as of the date you represent you are small in the initial offer that includes price. Your SAM primary is a self-selected classification of your main line of business. Evaluators and the CO look at the solicitation NAICS and your size representation against that standard. They are not limited to your primary row.

Listing several NAICS codes that you actually perform does not itself sink a bid. What creates trouble is listing a code you do not work in, or representing small under a size standard you do not meet. For SBA certification programs other than 8(a), 13 CFR 121.404(d) looks at whether you are small under any NAICS code listed in your SAM profile, so keep those rows honest. For an ordinary small business set-aside RFQ like the admin support one your prime asked about, leave 561320 as primary if staffing is still your main work, keep 561110 on the list if you perform it, and certify size against the solicitation's 561110 standard.

When I renew SAM for clients I put their real revenue-driving code as primary, add every code they actually deliver under, and drop anything that is just aspirational. That keeps the profile clean without changing which RFQs they can answer.