Short answer: It depends on the state and carrier. Many carriers require at least two enrolling people (such as two owners, or an owner plus a W-2 employee); some accept a “group of one” partnership. A solo owner with no employees usually buys individual coverage instead.
Small-group eligibility generally requires at least two eligible, enrolling individuals, for example two non-spouse owners, or one owner plus a full-time W-2 employee. Rules vary: some carriers accept husband-and-wife groups or a partnership/LLC “group of one,” while others don’t. A sole proprietor with no employees typically can’t form a group and would shop the individual market (or use an ICHRA from another employer). Always confirm the carrier’s owner-eligibility rules.