It is true that there are more jobs in maths than in physics, but quantity should not be your only criterion. Every pupil studies maths, but not all take physics. Which type of post is likely to provide you with teaching at ASA-level? New teachers can be given challenging timetables with little or no groups containing talented or gifted pupils. This is less likely with physics. With physics you can also probably negotiate which other subjects you will teach more easily. And promotion may be faster.