The OTO limit for the Windows version of UTAU is 2^15 or 32,768. This is the total number of lines of OTO you can have in an entire voicebank before it no longer functions. While it was uncommon to reach it in the past, you are more likely to reach it now due to things like multipitch English voicebanks. (Yes, multiple pitches/appends/etc count towards the limit if the bank is used all together. If you separate each append/etc into a separate voicebank and use them separately, their OTOs don't add up.)
In UTAU-Synth on Mac, I've been able to reach 2^20 lines, or 1,048,576. It's probably even higher still, but at this point, it was starting to lag a lot on my poor old Macbook Air.