We had a entity with 276 attributes (don't ask why). This worked for us, this was a postgresql db. So 100 attributes in your entity is not a limitation in the modeler. See this for more info on MS SQL
At the moment we do have an entity with approximately 170+ attributes (questionnaire as well) and are using the same application database. We haven't experienced any problems.
SQL Server has a limit on the total amount of characters you can use in column names in a particular table, so yeah you can make an entity with 100 attributes but don't make the attribute names too long.