$6,000.

Top of page 4:

https://www.irs.gov/pub/irs-pdf/i2441.pdf

"To qualify for the credit, you must have one or more qualifying persons. You should show the expenses for each child in column (c) of line 2. However, it is possible a qualifying child could have no expenses and a second child could have expenses exceeding $3,000. You should list -0- for the one child and the actual amount for the second child. The $6,000 limit would still be used to compute your credit unless you have already excluded or deducted, in Part III, certain dependent care benefits paid to you (or on your behalf) by your employer."