Not quite sure if that's what you are after. I assume that you've
created a join between you list.
The following should get you started.
Create a new data  form web  part and choose some attributes from the
master (projects) and insert them as multiple items. This gives you a
list with your projects  (guess that's where you are at the moment).

When done decide, where you want to include the detail information (e.g.
create a new colum or new row), choose some attributes from the child
(project tasks) and choose Insert Selected Fields As Joined subview.
This will give you an option to determine the join field.
If you look at the source  code SPD produces you'll see a new XSLT
template dvt_2 with some variables defined at the top.
<xsl:variable name="dvt_ParentRow" select="current()" />
<xsl:variable name="Rows"
select="../../../Tasks/Rows/Row[@Project=$dvt_ParentRow/@Title]" />

If you are not interested in showing  the task details, but only count
them you can modify this template to reflect your needs.

<xsl:variable name="dvt_ParentRow" select="current()" />
<xsl:variable name="CountTasks"
select="count(../../../Tasks/Rows/Row[@Project=$dvt_ParentRow/@Title])"
/>
<xsl:value-of select="$CountTasks" />