Let's try to be quantitative, despite the unknowns.
For the next 3 years until 2024, RYR expect to grow by circa 115 aircraft. That's data from their official website, taken at face value. Just what you would expect, given the almost sealed fate of some of their infirm competitors. It will be survival of the fittest - and RYR are in a position to take advantage of the failure of many other airlines.
For a LCC, the usual ratio is 6 crews per aircraft. 115×6=690 new FOs needed over 3 years for expansion purposes only. Possibly more as some of the existing ones will move onto command, while others will swap companies (this one towards the end of the period though, right now nobody is going anywhere and won't be for another year or two).
And here comes the hardest part... How many people are in the pipeline now? Without any good estimate of that, we won't be able to figure out the rest.