At the end of the trial period, the addons will move to Active state. When it does, a separate invoice is generated for the addon.
From the date of activation to the date of subscription renewal, a prorated charge will be generated for the addon.

Say, for a subscription with a billing cycle from January 15 to February 15, an addon was added as a trial on January 20 for 10 days. The addon will activate on January 30 and an invoice with a prorated charge will be raised for January 31 to February 15.

If the customer's subscription renews on the same day as the addon is activated, they will receive two invoices, one for the subscription and the other for the addon, irrespective of the number of addons.

Say, the customer has a subscription and has 3 addons on trial. Subscription renews on the same day as the addons become activated. The customer will receive 4 invoices, one for the subscription renewal and the other 3 for the addons.

Note: A separate invoice will be generated at the end of add-on trial even when Consolidated Invoicing is enabled for your Chargebee site.