Can we use the present perfect and the past simple interchangeably here, please?
"I don't remember, I've lost/lost count!"
Not interchangeably, but either might be appropriate. 'I've lost count' is probably appropriate more often (in the UK, that is). But you could say 'The announcement said the train was due in fifteen minutes and I thought I'd try to count the seconds. But I lost count after about five minutes.'