Once your employment ends, you won't be able to spend your FSA funds, but you do have 90 days to submit claims for FSA-eligible expenses that you incurred while employed and during the current plan year. The Flexible Spending Account app will still appear on your dashboard in order for you to submit claims. Any unused money remaining in your FSA at the end of the plan year is returned to your employer. If you're called to active military service during your FSA plan year, though, you'll be able to cash out your FSA balance.
Multiple reimbursement claims can be submitted at the same time as long as all required documentation is supplied.