The FAQ says "Electrum uses a gap limit to stop looking for addresses, it is set to 5 by default, so the client get all addresses until 5 unused addresses are found."
It doesn't say HOW-TO actually increase the number of addresses.
Anyone?
1. open console tab
2. type in "wallet.storage.put('gap_limit',x)" without quotes around it, and replace x with the number of addresses you want it to create for unused addresses. (ie. if you have 5 unused addresses now, and you want to make 3 more, set it to 8. From then on, every time you use an address, it will create a new one so that you always have 8 unused addresses showing.)
CAUTION: The way that Electrum works is to search for the balance in all addresses within the gap limit, so if you set the gap limit to 500, and use the 500th address to store bitcoin, you will have to set your gap limit to at least 500 every time you restore your seed, otherwise Electrum won't "see" your 500th address correctly. Because this is impractical, please use addresses in your Electrum wallet in the order that they were generated, and try not to increase your gap limit too high, as it will be cause for confusion in the future if you ever have to restore the wallet from seed.