According to the data posted on the official website by MyHeritage, you can add 250 people in the tree and the limit of storage space is 500 MB. This is for the Basic package and it doesn't matter if you used the Family Tree Builder on the desktop PCs.
However, the Premium plan allows you to add an unlimited number of people and you also have additional storage. The Premium plan is billed yearly. For additional information, consult the website. Also, if the MyHeritage Family Tree Builder is installed on your PC, then it should use the same subscription plans.