Is SPF 50 foundation enough without sunscreen?

Usually, makeup is not applied in the amount needed to provide its labelled protection. The issue is amount: most people do not apply foundation at the same thickness used for sunscreen testing.

SPF in makeup is difficult to apply in sunscreen-level amounts

Use a dedicated broad-spectrum sunscreen as directed, then makeup if you like. SPF in foundation can be an extra, but it should not replace proper application and reapplication of sunscreen. Choose a combination that you can wear comfortably and consistently.