How many times can pleural effusion in a baby be drained?

As many as needed: As many times as needed until correction of the causative process can be addressed. Question is too inespecific to give context to a more thorough explanation and answer.

No limit: Generally, no limit on "how many times". However, the more times someone undergoes an invasive procedure the greater risk of complications. Key to preventing recurrence is answering "why the baby developed an effusion?". Then treat the primary cause. Be well.