I finally unlocked this achievement after purchasing all of the bikes. The description made it sound as though it was just one of each type so I thought it may have been glitched but I had to buy all of them for it to unlock. This will take a couple of million in game currency. There are 20 Supersport and 20 Superbikes.
(I was at 337,000 fans with 356 wins before I got this, to give you some idea of how long it could take.)
To unlock the bikes you need to complete the manufacturer invitational events. When selecting them for your career make sure to check if you need it first, as it does repeat the events, For me, it took 5 seasons before I got the Norton invitational (for some it has appeared in the first season), so if you are missing any they will show up eventually.
Handy tips: If a race is available for both Supersport and Superbike, you only need to do it with one or the other to earn the reward. I spent ages doing it with both trying to earn money, this could save you a lot of time.
You don't have to do every event, after you have done a few seasons and earned some fans, you will start getting much more high value races, some of these are quite short events, so check your calendar and do the best events based on time versus reward. If you find a race you chose that you don't want to do, skipping it will only cost your entry fee, you won't lose any fans.
Completing a manufacturer invitational earns you 20% off your next purchase from them, use it for the most expensive bikes to maximise profit. If you own all of their bikes, you can sell yours for full price then re-buy at the discounted rate.
Possible helpful glitch
As I was going for the final bikes, I had completed the Suter Invitational again, so I could sell it then buy it back and make £20k profit. I couldn't get it to sell but it turns out it had given me the full sale price without it leaving my garage. I tried some other bikes and around 2/3 of my superbikes sold without leaving my garage, leaving me with over £1m spare cash. I had 19 supersport and 19 superbikes in my garage when I tried this, not sure if it would work with other amounts. I tried selling the last superbikes towards the bottom of the list when this glitch happened. (The supersports all sold as they should have)
Seeing as you don't lose out anything selling then re-buying, it's worth periodically trying to sell some bikes to see if this works, as this is a very grindy achievement.