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

I believe this is going to be based on the size of your connection pool in the database itself

Thanks Aleski...

Could you please elobrate connections with below example.

Suppose, there are 1000 process are running, write to multi datastore adapter is configured in the process. suppose if database has 20 sessions (i.e. connections), will Appian Data store adapter will make 20 connections when multiple processes are running

Data source pool size specifications are specified in appian-ds.xml file which will be present in location(JBOSS_HOME/standalone/deployments/appian-ds.xml). So under <xa-datasource> tag you can find <xa-pool> tag there are setting for max and min pool.

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.