You should definitely get some more, and ASAP because SOX18 is hard to find and skyrocketing in price! I have a Philips SOX 18W outdoor light that burns around 3000h/year, and while its original lamp from 1971 did 5 years, the more recently made lamps only seem to last about 2y, which is not long at all.
The lamp in your photo is a good old Philips which seems not heavily used and should hopefully work for its rated 18,000h. But the newer ones seem to be terrible. Also thanks to Philips’ technique of gettering the arc tube with blackened ends around the electrodes, which is good for long lamps, this blocks far too much light in the short 18W and they are notably dimmer than all others. I tried one GE lamp which only did 18 months, so now I am running an Osram to see if that will do better.
My best advice when you search for spare lamps is avoid the newer Philips if you can, and in general buy the oldest lamps you can find which will almost certainly deliver much better life and light output.