For articles with more than 7 authors, is it compulsory to shorten it or just suggested?

This depends entirely on the style sheet you are using. For example, MLA demands "et al." for three or more authors; APA for six or more. The right thing would be to familiarize yourself with the house style of the publication you are targeting.

A small addition to @henning 's answer which I agree 100% with:

sometimes, depending on a particular journal and style guide, it also depends whether the citation is in-text or in the list of references, and even the referenced publication origin. In some cases, the style guide phrasing can be suggestive, and for some – compulsory.

Moreover, different journals have different standards on applying their recommended or compulsory guides in practice.

Take IEEE Transactions as an example.

For in-text references, according to Section IA of IEEE Reference Guide (2018):

NOTE: Use et al. when three or more names are given for a reference cited in the text

For the reference section, according to Section II of the aforementioned guide:

If there are more than six names listed, use the primary author’s name followed by et al. For non-IEEE publications, et al. may be used if names are not provided.

With this example, I wanted to demonstrate that the rules can be very different and unexpected; therefore, one must get familiarized by a style guide used by a particular journal, and maybe some recommendations that are specific to only this journal.

Presumably you are going to submit a paper to some journal. It may be that they have their own rules for this, so you should check. And, it may also depend on the field.

There are publications in some sciences in which the list of authors is longer than the article itself. Things done at CERN, for example might have this characteristic.

And whether you use ellipsis as you show in the example, or the first (couple of) authors with "et al." also depends on the field depending on how important the last named author is.