You can follow the workaround to add more than 250 cart line items.
This workaround is for Digital Commerce Turbo implementation only.
You can test the workaround with additional 50 cart line items. Before you do that you need to change the page size from 250 to 400 in the DCTurboConnection class.
String response = this.doGet('/pricing/carts/' + cartId + '/lines?includeAdjustments=true&pageSize=250&getResponseOnPriceComplete=true' + lineQueryParam);
String response = this.doGet('/pricing/carts/' + cartId + '/lines?includeAdjustments=true&pageSize=400&getResponseOnPriceComplete=true' + lineQueryParam);
Now, start by incrementing with 50 cart line items first. Test it on your org. If the test is successful, test further with another 50 cart line items. Continue to add additional cart line items until you hit the Salesforce system limitations. Depending on the complexity of items (number of constraint rules at bundle or cart level execution or too many fields/custom fields on each line item) in your cart the maximum number of line items may vary.
This may or may not work depending on your org limitations.
Alternately, based on line items count you can pass the pageSize and pageNumber dynamically.
For example, if you have 375 line items, try the following:
pageSize=250&pageNumber = 1
pageSize=175& pageNumber = 2.