初中英语必掌握的短语数量并没有一个统一的标准，因为不同的教材、教学大纲和考试要求可能会有所不同。然而，根据一些资料，我们可以大致了解初中英语中常见的一些短语。

感官动词+do

see, hear, notice, find, feel, listen to, look at

比较级 and 比较级

例如：bigger, more interesting

常用短语

a bit of, a large number of, at the same time, look forward to, a bottle/glass of, a farm worker, a line of, piece of cake, call back, take care of, make a choice, etc.

介词短语

in front of, on the, after, at, by, for, with, about, against, between, into, through, during, before, after, above, below, from, up, down, out, off, over, under, again, further, then, once, here, there, when, where, why, how, all, any, both, each, few, more, most, several, some, such, no, nor, not, only, own, so, than, too, very, can, will, just, don, should, now

动词短语

give up, look after, look forward to, pay attention to, take care of, get along with, make friends with, break down, find out, come across, come up with, deal with, pay for, put off, take place, belong to, break the rule, take place, at ease, in a hurry, on the other hand, by accident, on purpose, take place, take care, keep on, get tired, keep in touch, be familiar with, find, join, make a decision, make progress, lose heart, be successful, be afraid of, be interested in, be busy with, be free from, be fond of, be angry with, be patient with, be proud of, be satisfied with, be busy doing, be tired of, be busy with, be free from, be fond of, be angry with, be patient with, be proud of, be satisfied with, be busy doing

句式

简单句（主 + 谓 + 宾）

并列句（and, but, or）

复合句（主句 + 从句）

综合以上信息，初中英语必掌握的短语数量可能在几百到上千个不等，具体数量取决于教学大纲和考试要求。建议学生根据自己的实际情况，选择合适的教材和资料，系统地学习和掌握这些短语。