There is no limit in the number of connections, per se.

However, there are such requirements as "most direct routing" and published MPMs or routing rules that award tickets are subject to. (Edited to add that it may be possible that the SIN routing is not permitted on SFO-HKT published MPMs/routings).

It sounds to me like your first agent did not look at all the possibilities at first - it's quite possible since the consolidation of the revenue and award international desks: revenue agents weren't as resourceful at looking at all possible routings as award agents were...

I would certainly call and see if you can have your entire preferred routing booked as one award.