"按行数支付" can be translated as "payment based on the number of lines" or "payment per line" in English, depending on the specific context and intended nuance.
"Payment based on the number of lines" is more descriptive and clearly explains that the payment is calculated according to how many lines there are.
"Payment per line" is more concise and commonly used, especially in contexts like freelance writing, coding, or data entry where payment is often calculated per line of work completed.