The maximum or minimum match for a Turnitin assignment report varies from assignment to assignment and is at the discretion of your teaching staff.
A literature review will generally have more quoted content than an original case report, for example. Contact your lecturer in the first instance to see if they have any set limits or advice.
Assignments with a report that shows a high match percentage to external content will get individually checked to make sure that you are referencing correctly.
Just remember to cite your material correctly. No amount of genuine plagiarism or non-cited quotation is allowable in any assignment.