If the network already exists and has IPX running on it, you need to use the same number that's already on it; contact whoever assigns IPX network numbers in your organization, because you can cause connectivity problems if you just invent something that clashes with the existing scheme.
If this is the first IPX device on the (only)network, make something up. It needs to be a hexadecimal number of 8 or fewer digits. BEEFBABE, ADEADC0D, 1, C0FFEE, and 90210 are all perfectly valid IPX network numbers, for example. All workstations will require the same number.