How many costing strings can be entered when creating a requisition? Is there a limit?

No, there is no limit.  But here is a consideration.

When setting up a requisition, a hiring manager can “split code” it to an unlimited number of costing strings, to allocate the costs to multiple funding sources. These code combinations will “flow down” to the associated timesheets and be available for time reporting, but there will be no automatic allocation of the worker’s time across the funding sources. The contingent worker will need to be guided to report time correctly against each available funding source.