I'll go out on a limb here, make a possibly unwarranted assumption, rush in with no concern for my personal welfare to a place where angels fear to tread, and presume that you're talking about the angle formed by the hour- and minute hands of an old fashioned clock. On that assumption, I count eleven (11) of them.
11 Two right angles are formed during each hourly period 4-5, 5-6, 6-7, 7-8, 8-9. However, when the clock reaches 9pm, this coincides with the second 90° between the hours of 8-9. The clock has then to reach approximately 9.34pm for the only occasion when the hands will be at 90° in the hour 9-10.
16:00 is 4pm on a 24 hour clock.
Hours are between 4pm and 9pm is 5 hours, 9 pm -4 pm
32 - 15 = 17 per hour
That is 8.5 hours
Usually between 3 and 4pm