I thought there might be a few more Bulgarians in the country than that.
Since it's essentially variants of the same language and culture hanging on to a Bulgarian identity for generations hasn't been that common, you'd need to actively prevent intermarriage with Macedonians and gradual assimilation, and if you add the constant pressure from the authorities and Macedonian society in general it's no wonder there aren't many left who identify as Bulgarians. There was some immigration of Serbian Bulgarians to Macedonia which kept the Bulgarian identity alive for a bit. It's mainly around Strumica in the SE that you've had enough Bulgarians to retain a Bulgarian minority identity. The Macedonians who have applied for Bulgarian citizenship is a much bigger group, but that was mostly to be allowed to work and live in the EU.
For comparison there are 90k Slovaks in the Czech Republic, but their separation is much more recent than the current North Macedonia being separated from Bulgaria, they haven't been united since they were both part of the Ottoman Empire.