While your plan offers you the option to borrow, you need to be aware of any restrictions put in place by the plan administrator. Such information may be found on the administrator website or in the plan's summary document available from the administrator or possibly your HR representative.
Generally, if the plan allows loans, the rule is you can borrow up to 50% of your vested balance to a maximum of $50,000 IF you had no other plan loan in place in the 12 month period ending on the day you apply for the loan.
In your case where you have a plan loan outstanding, then the new loan is limited. It is the lesser of 50% of the vested account balance or $50,000 minus the outstanding loan balance in the preceding 12 months.
In addition to the minimum loan amount noted previously, your plan administrator may restrict the reason for the loan to a list of approved reasons (for example, uninsured medical expenses, college tuition, purchase of a primary residence).
Just because you can get a loan from your 401k doesn't mean that you should. Sure, it is convenient, fast and relatively inexpensive. Yes, you are "paying yourself" for the borrowed money and not a credit card company or bank. Yes, the interest is tax-sheltered and there is no tax consequence for receiving funds as long as you pay it back.
But if you are nearing retirement, may be changing jobs or are facing a lay-off in the near future, need the proceeds for meeting daily living expenses (as opposed to some extraordinary or large capital expense) or are using your 401k like a piggy bank to buy luxury items or pay for a vacation, then the answer is simple: Don't do it.
By regulation you can borrow up to $50,000 or 50% of your current account value which ever is less. You will need to contact your plan administrator to determine if your plan allows it.
Some plans may offer more than one loan at a time, and usually a loan minimum of $1000.00 As Tony mentioned, your plan administrator will be able to guide through this process.