I get asked this question often from small businesses. My answer is, "As many as accurately describe the actually product(s) or service(s) you can provide."
Some firms choose dozens of NAICS codes using a scatter-shot approach. But what impression does that give the Contracting Officer (CO/KO)? They are thinking (at least) two things:
▶️ "There is no way this small business does all this."
AND
▶️ "What portion of all these NAICS can they really provide?"
Some industries lend themselves to many NAICS, like IT and Real Estate. Listing all related NAICS is a good strategy if you actually perform several types of interrelated services that fall under separate NAICS.
But other industries don't. In those cases, having a mish-mash laundry list of NAICS will have the CO/KO scratching their head wondering if your business has chosen a lane, knows what that lane is, and is experienced and focused enough in those NAICS to provide the products or services it says it can to an acceptable standard.
Then factor in SBA Dynamic Small Business Search (DSBS) and narrative portions of that record. With DSBS being the authoritative source repository for SBA small business certifications, every CO/KO goes to it at some point in their market research. Does your record capture info related to your chosen NAICS well enough that the CO/KO will want to open your Capabilities Statement or hit the link to your website to learn more?
I've read hundreds of DSBS records where narratives are over-generalized, don't use the space provided, and lack good use of key words. Even if your profile lists the NAICS code the CO/KO needs, if the DSBS narrative or record does not speak to the NAICS listed the CO/KO will be left wondering if you are a viable contractor for the work. They have no info from which to document market research affirmatively about your firm. You'll end up counted in the market research as a company who has the NAICS code but not counted as a firm the CO/KO believes could likely submit a successful offer.
Why? Well, market research isn't just about how many small businesses claim they can do work under a NAICS code on their SAM, DSBS, or RFI response or hold a certain socioeconomic certification.
It's all about the CO/KO having "...a reasonable expectation of obtaining offers from two or more responsible small business concerns that are competitive in terms of fair market prices, quality, and delivery", or in other words, a proposal likely to win. If you're all over the map in your NAICS codes and don't capture them in your DSBS narrative, the CO/KO won't necessarily open your capabilities statement or go to your website because they won't have confidence that they will receive a competitive proposal from you.
Bottom line: Don't over-select NAICS. Use the space you're allowed in DSBS to talk to all your selected NAICS. Use DSBS fields to the max. Make your record count! It can be your first and only impression maker!