Finge Benefit Plan Count

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For large plan participant count purposes, are number of participants in a medical spending account based on the number eligible to participate or the number participating in the plan (had at least $1 excluded from gross income).

Number example: 1000 eligible employees, 50 employees elect to have salary reduced for medical expense reimbursement. Is a 5500 required to be filed?

As always, appreciate your comments. Thanks