Should you have a co-founder? Probably yes — if you can find the right one. Co-founded startups raise more capital, ship faster, and survive longer than solo-founded ones, according to YC, Stripe Atlas, and most pre-seed VC data. But a wrong co-founder is worse than no co-founder — the equity split is hard to unwind, the strategic disagreement is hard to resolve, and the relationship is the most exposed surface area in any early-stage company.
Solo founder works fine for productised services, indie SaaS, content businesses, and most lifestyle businesses. It works less well for venture-track startups requiring fast scaling, multi-discipline products (where one person can't credibly own both eng and GTM), and capital-intensive plays.
If you do have a co-founder: write a co-founder agreement before incorporation. Equity vests over 4 years with a 1-year cliff. Decision rights are explicit. Disagreement resolution is named.