I understand that it would depend on which beach head (Omaha, Utah,ect) but is there an overall estimate of the German forces?
Hard to tell. Most of them were not directly defending the beach if you are asking that.
4 Major units were present in that area.
The strongest unit was the 21st Panzer division with about 16.000 men
The 352nd Infantry division had ~13.000 men
The 709th Infantry division had 12,320 men
The 716th Infantry division had 7,771 men
This would give the raw number of roughly 49,000 Wehrmacht personnel ( in divisions) in that area. Of these a good chuck is divisional support troops and another big chunk was held behind the beaches as reserves and defending strong points.
Normandy 1944: German Military Organization, Combat Power and Organizational Effectiveness
How many of them were directly defending the beaches and the close area behind?
The defense doctrine for the beaches required a substantial part of the forces to be deployed as reserves only a small part of the troops were positioned in the beach area even less were facing the beaches most were behind the beach to be able to counter potential breaches. Most troops were far behind the beaches around the bigger cities.
To give an example of the deployment of forces.
This sector was defended by the 352nd and parts of the 716th. One Regiment of the 352nd was kept in reserve and the other two Regiments had only parts of their forces in the beach area the rest some miles behind.
How many of those were literally aiming on the beaches or supporting those who did?
Zaloga lists 12 "Widerstandsnester" ( literally resistance nest ) little defense strong points facing the beach. 20-40 men per WN seems to be a reasonable estimation. The design of the WNs varied a lot. This would give around ~250 men facing the Omaha assault since some of the WNs had no direct view on attacking forces and were either positioned to defend the draws or just had no view on the beach were the assault took place.
Campaign 100: D-Day 1944 (1) Omaha Beach
After the first line of defense was breached more units got involved many got destroyed when they tried to reinforce the beach but were suprised by enemy forces.
This is an approximation on my part but over the entire landing front i would guess around 1500 men were facing the Allied troops directly on the beaches.