You can generate the addresses using this
https://dcpos.github.io/bip39/You can generate as many addresses as you like. If you know the last address your Mycelium HD wallet generated, then you only need to generate addresses until you reach that one.
If you don't have access to your Mycelium HD wallet, just keep generating addresses and checking them against the blockchain until you reach an address that has never been used.
I tried it for my wallet, which I have been using long enough to generate more than 200 addresses.
I opened the converter, offline of course, typed in the words and clicked Mycelium. It reached the last address of my wallet after only 84 addresses. Probably the reason for this is that the converter only generates the receiving addresses, not the "change" addresses.
If somebody can explain why the converter generated fewer addresses than Mycelium in actual use, please pipe up.
Naturally, you can find the change addresses to your receiving addresses by looking on the blockchain.