I have a Cms version 6.4.5 with Forms version 5.1.66.
I would like to update the Cms to version 188.8.131.52. Is it necessary to update Forms too?
I would like to avoid updating Form.
According to official release notes, the latest AEM 6.4 is 184.108.40.206. In terms of AEM upgrade, yes you will need to upgrade AEM Forms as well. Assuming that you would like to upgrade your AEM to version 220.127.116.11, you will need AEM Forms to be upgraded to version 5.1.212.
I do not think you can skip AEM Forms upgrade - you can try on your own, but even if it will work, this is not recommended approach.
Please find detailed AEM and AEM Forms compatibility matrix to get all information about AEM Forms version you will need upgrade to, depending on AEM version.
All useful links in context of this topic:
by carrying out tests I find that it is also necessary to update the Form for the correct functioning of the Cms.