It depends on when, for how long, how far the heartbeat drops, really. Plus other factors need to be taken into account.
There are some 'decelerations' that are more worrying than others (and sometimes it's hard to work out what's a worrying decel and what isn't, which is why CTGs are notoriously unreliable, but anyway...!)
In a nutshell:
'early' decels that start with the contraction and end with or before the end of the contraction are associated with normal fetal head compression as the baby moves down the birth canal and are not usually regarded as a problem. Usually all that is required is a change in maternal position/posture.
'late' decels that reach their lowest point after the peak of the contraction are associated with more prolonged lack of oxygen to the fetus. They usually mean that the fetus is not coping so well with the contractions, etc.
A prolonged decel (a drop in the fetal heartrate of more than 30beats per min for 2 minutes or more) may mean e.g. cord prolapse or other problem.
This is a REALLY basic explanation! Obviously when you're interpreting a CTG you should look at all the variables - length of labour, risk factors, etc etc etc.
As for the 'highest' number, I think our machines go 100+ but I've always been told that the actual number doesn't really mean anything. The length, frequency and strength on palpation is what counts, not the figure on the machine.
HTH!