I think having more teams gets you more development of the non-tops. I think you can safely bet on 8 to 10 countries making it into the top 16. For all the countries to get a shot means they will want to work for that final spot because there's a better chance of actually getting there. This doesn't mean San Marino or Gibraltar, but you do get Wales, Finland, Latvia etc... getting on mid-Europe level in best case scenario.
If Great Britain were to compete as one, like in the Olympics, you lose 2 or 3 of these countries that have a chance.
All in all, I think the 24 team or 32 team format works great for large pools, Europe might be too small to actually make 32 or 28 work. 24 seems high but 16 seems low.