how much amount of emails supported by EXM by default?

There are two things that come from your question, the first thing is the overall limit of the EXM for all the recipients and second is maximum email messages that can be sent to a contact or a single recipient.

First, the overall limit of the EXM, there is no limit on sending email in EXM. It all depends on the SMTP Server configuration and limits. From the Sitecore side, there is no limit for email messages that can be sent to all the recipients.

Second, the email messages that can be sent to a contact, for this you can define and enable the sending limit in Sitecore content editor. OOTB there are three parameters you can define for the sending limit i.e. Daily send limit, Weekly send limit, and Monthly send limit. you can define this on each manager root. see the below image -

EXM uses the server date when verifying if the number of sent email messages to a contact exceeds the sending limits. You need to select the Send Limits Enabled checkbox to activate the sending limits. In the Daily, Weekly, and Monthly Send Limit fields, define the maximum number of email messages that can be sent to an individual recipient. If you set a limit to zero, no email messages are sent. If you leave a sending limit blank, an infinite number of email messages can be sent.

The important thing here is that if there any inconsistencies across these fields then the most restrictive rule will work. for example monthly limit is 10 and weekly is 5 then it will not send email in 3rd week if already process the 10 emails.