How to doOnNext only consumes the first n items of a PublishSubject?

I have a PublishSubject:

val myPublishSubject = remember {
    PublishSubject.create<Long>().apply {
        doOnNext {
            Logger.debug(TAG) { "Got new Long $it" }
        }
            .takeUntil(someObservable)
            .subscribe()
    }
}

and I have a service what gets a stream of Long values, somewhere in another function:

while(notFinished){
    val newLong = getSomeLong()
    myPublishSubject.onNext(newLong)
}

Here, the upper doOnNext{} is logging forever. How can I for instance only allow first 20 Long values? .take(20) did not work!

take(20) should work, this test passes:

@Test
fun consumeOnlyFirst20ValueTest() {
    val ps = PublishSubject.create<Long>()

    CoroutineScope(Dispatchers.Default).launch {
        for (i in 1L..1000L) {
            ps.onNext(i)
            delay(50)
        }
    }

    var i = 0
    ps.take(20)
            .doOnNext {
                i++
            }.subscribeOn(Schedulers.io())
            .subscribe()

    Thread.sleep(6000)
    assertEquals(20, i)
}