How many times does an output transformer need to be interleaved?

An output transformer does not need to be interleaved a “specific number” of times – the optimal interleaving depends on the application and the desired quality level. By interleaving the primary and secondary windings, coupling is improved, leakage inductance is reduced and the frequency response, especially in the high-frequency range, is optimized. Typical values are 2–3 sections for standard applications and around 5–7 sections for high-quality HiFi and tube amplifiers.

However: Too many sections can increase the winding capacitance and make the construction more expensive.