is there a limit on # connections Appian (v 7.3) can make with database ? if so,

I'm not entirely sure on how it would work in that situation, but my assumption would be the following:
If 1000 transactions need to be made, and each transaction needs its own connection, then all the available connections for the database will be used. The remaining transactions will wait until an active transaction completes and frees up the connection, which it will then use. If all these are running at once then that might cause a backup and performance degradation as all these pending transactions are trying to use the database at once.

I'm not entirely sure on how it would work in that situation, but my assumption would be the following:
If 1000 transactions need to be made, and each transaction needs its own connection, then all the available connections for the database will be used. The remaining transactions will wait until an active transaction completes and frees up the connection, which it will then use. If all these are running at once then that might cause a backup and performance degradation as all these pending transactions are trying to use the database at once.