From looking at the sources below, it looks like Ukraine still has about 1/3 of the fighter aircraft it started the war with, though it started with many non-serviceable units (seems that at least 20 aircraft were non-operational), and received many parts from abroad:

https://en.wikipedia.org/wiki/Aerial_warfare_in_the_Russo-Uk...

https://aerospaceglobalnews.com/news/how-many-aircraft-losse...

I am not sure what is meant by 'a significant number of', and I'm not sure if all commenters have a common definition of that phrase, so I'm unable to judge the veracity of the comments above.

Pretty sure almost all of the functional ones were destroyed or were kept in such reserve for security they weren't being used.