There is a hard limit of 4096 columns in a MySQL table. Each site in MSM creates additional columns in the
exp_channel_data table: 2 columns for each custom field. If you work out how many custom fields you have, double that number and divide it into 4096 and round down you will end up with an approximate maximum for the number of sites that is possible:
Maximum no of sites = 4096 / [No of custom fields] * 2
Note that other factors can effect the maximum number of columns permissable, such as the total allowed row size in a MySQL table (65,535 bytes) 
For this reason, MSM is not really a scalable solution to the problem of multiple sites which share data. You might be better off sharing data with a parent/base site via an API .
Alternatively, you could investigate assigning fieldgroups created in site 1 to channels created in site X, so that those database columns are reused rather than duplicated. This can be done by manually editing the field_group value assigned to a channel in the
exp_channel table. Surprisingly, the CP publish/edit forms work as expected with this change. However, I haven't investigated how this would impact tags used in front-end templates.