The Jedi have had a not-so-subtle Sith Lord sitting right in front of their noses/nostrils/snouts/gills for about 15 years and didn't notice a thing!
So what do
they know!?
"Always two there are!"... yeah right.
Palapatine should keep a ready supply of candidates ready to go, in case the apprentice gets knocked off (of if the Master is killed).
Who says he didn't? There are these Inquisitors, who are active, lightsabre-wielding force users. Since they are in the animated series "Rebels", they are official canon. It might be possible that they were trained by Vader...or that Palpatine did have some potential Sith apprentices hidden away somewhere.
The fact that he was so keen on recruiting Anakin was due to the prophecy of him being the Chosen One (tm), and very strong in the Force (high Midichlorian count - thanks again for that, Mr. Lucas
). It does not necessarily mean that he didn't have a plan B, or C, or D....