No. Three years is the maximum per admission; TN can be renewed repeatedly as long as she remains a Canadian citizen in a listed profession with a prearranged engagement and continues to have temporary intent.
A ceiling on each admission, not on the classification
The TN regulation admits a professional for the period of the engagement up to three years and allows extensions in increments of up to three years, without stating an overall maximum. The classification requires Canadian or Mexican citizenship, a profession listed in the USMCA professionals appendix, the credential that entry specifies, and prearranged professional employment with a U.S. employer; self-employment is not permitted, and any ownership stake in the employer must be disclosed.
Physiotherapist is a listed profession, and the credential is a state or provincial licence or a baccalaureate degree; she holds both. What does limit the classification is intent. TN is not a dual-intent category, so an applicant who has filed for permanent residence, or whose ties to Canada have faded entirely, may be questioned at renewal.
A career of TN renewals is possible, but each is decided on that day’s facts, and a permanent-residence plan should be sequenced so that it does not collide with a border renewal. A first review should separate eligibility from presentation. Match the offered duties to the listed profession, then compare the degree, licence and employment letter against that description; the employer letter and current credential normally settle the point.
Decide before travel whether a port-of-entry application is appropriate or whether the employer should use Form I-129 for an in-country extension. A contractor who controls the work, invoices through her own business, or holds herself out as the employer cannot cure self-employment by calling the arrangement temporary. Hypothetical example: a medical laboratory technologist is offered a salaried role at a hospital laboratory, but her draft letter says she will bill patients independently.
Revising the actual employment arrangement, not merely the wording, is the decision that determines whether TN is available.