They were numbered 35x or 36x in line with the rest of the 3xx Bristol Country routes, with a few exceptions such as the X1 (WSM-Bristol limited stop via Long Ashton bypass) and X7 (Clevedon-Bristol limited stop via Failand).
If only it was that easy!
I'm sure that TGW will fill in any bits I've missed however the whole thing has been a combination of First's love for tinkering and some OCD from their marketing department. The Bristol country services were renumbered in 1967 into the 3xx series and the services to Weston/Clevedon/Portishead/Nailsea were all in the 35x and 36x series. The only real change to this over the years has been the arrival of the X1 which, as the X suggests, was an express route.
At some point First decided that they'd renumber the Portishead services as X2 and X3 as a big launch with lots of publicity. (The X being related to the marketing message 'Express yourself, but not using the initial letter obviously).
At some other point somebody decided that the Weston service being the 352 wasn't exciting enough so they renumbered it the 1 (because the express version was the X1). Some time after this happened, and after numerous issues with people trying to get on the wrong bus, the message finally got through that having two service 1s from the same company leaving from the same stop (in Bristol city centre) but going to different places wasn't a great idea and the Weston service became the W1.
The X7 used to be an express services to Clevedon however it had been dropped at some point and First decided to change the number of their Chepstow service to use it (as it crosses the SeveRn)
Later on it was decided to market all of the services in the area as the 'Excel' network (but again First didn't like Es). The problem with this was that First's OCD cut in again so (to get enough numbers in a row) their existing X7 service was renamed as the Severn Express however this didn't fit their systems properly so, online, they called it the 7XP which you had to use if you wanted to find it for the rest of it's existence. They also wanted the Weston services together so the W1 became the X2 and the X2 became the X4. Despite this for the following two years several bus stops on the Portishead road had large posters advertising the previous versions that were left over from the previous marketing push!
Once the initial pattern for Excel had been setup First decided to start changing it however didn't have any more numbers they could use so services such as X3A and X1X started appearing so basically back to three digits but nothing like the previous ones.
Despite the number of years that have passed it's still possible to find lots of websites that refer to the previous numbers in their 'how to get to us' section, and Bristol City Council decided they couldn't be bothered keeping up so several of their bus stops still show the original numbers on the actual bus stop sign.
There are rumours that the buses on the Excel services are being replaced in the near future so I expect that there will be another round of number changes, because that's how to keep your existing users happy!