What is the next number?

𝐟𝐢𝐟𝐭𝐞𝐞𝐧, 𝐬𝐢𝐱𝐭𝐞𝐞𝐧, 𝐬𝐞𝐯𝐞𝐧𝐭𝐞𝐞𝐧, 𝐞𝐢𝐠𝐡𝐭𝐞𝐞𝐧, ...

• nineteen
• ninety
• twenty