It takes quite a few Specialists. Unless you happen to have the exact right set of buildings, you probably need three assigned to the same hex, and even then that often isn’t enough. You’ll need to have placed two buildings on the same tile and both buildings will need to be getting good bonuses from nearby tiles — per the Civilopedia entry for Specialists, they add +2 science, +2 culture, and also duplicate 0.5 the bonuses of the buildings they are on. If you don’t have any bonuses on the buildings you just get the +2 science and +2 culture, and even with three specialists on the hex that just gives you +2*3 = +6 science and +6 culture which even on a great tile is nowhere near the 40 you need for victory conditions.
So pile pairs of buildings, both of which receive very good bonuses on them from nearby hexes. Start putting on Specialists. If you’ve got a really great tile, you may get some of your victory points with just two Specialists on the tile. Keep an eye on the Tech Tree, there are several techs which increase the Specialists count. Late in the age if you’ve unlocked techs for it you’ll be able to get three on a tile. Look for your best tiles, stick on three specialists and you should be good to go. (Don’t forget if you’ve done a great job and gone over the 40 with just two specialists, don’t put a third on, if you’re trying to get the scientific victory, that third specialist is better of helping another hex get over the 40 limit).
