How making Projeqtor horizontally scalable ?

I am looking for an horizontally scalable project management

horizontal scalability is an infrastructure mean, not directly linked to application mean.
ProjeQtOr can easily be scaled : you can build Clusters of PHP servers and clusters of Database servers (I expect you will require the professional version of MySql or PostgreSql, not the free one)

I think it would be useful to use a nosql database in place of MySQL

No way.
Really no interest for this kind of application. Sql database have some advantages that may fit better needs of some management applications.
NoSql is not the good solution for all need.
Moreover, we want to rely on stable, easy to deploy infrastructure. I'm not sure OrientDb is as widely spread as MySql/MariaDB and as easy to deploy.

How numerous and difficult do you imagine they will be ?

Very very complex and long...

Do you plan to make them ?

Absolutely not.