Convert the entire amount. There is no limit on the amount that you can convert. You'll pay tax on the gain.
If you leave the gain in the traditional IRA, the $6,500 that you do convert will be slightly taxable and an equal amount of basis in nondeductible traditional IRA contributions will remain in your traditional IRAs to be applied to future distributions, so it doesn't make sense not to convert the entire amount so that you end up with a zero balance in your traditional IRAs at the end of 2024. Fidelity tends to deposit gains monthly, so there's a good chance that you'll still end up with a little bit of gain that you'll need to convert as a second conversion to be combined with the first.
